fix .Value problems

com.unity.uiwidgets/Runtime/painting/placeholder_span.cs

 namespace Unity.UIWidgets.painting {
     abstract class PlaceholderSpan : InlineSpan {
         public PlaceholderSpan(
-            TextBaseline? baseline = null,
+            TextBaseline baseline = TextBaseline.alphabetic,
             TextStyle style = null,
             PlaceholderAlignment alignment = PlaceholderAlignment.bottom
             
 
         public PlaceholderAlignment alignment;
 
-        public TextBaseline? baseline;
+        public TextBaseline baseline;
 
         public override void computeToPlainText(
             StringBuilder buffer,

com.unity.uiwidgets/Runtime/painting/text_painter.cs

 
     public class TextPainter {
         InlineSpan _text;
-        TextAlign? _textAlign;
+        TextAlign _textAlign;
         TextDirection? _textDirection;
         float _textScaleFactor;
         Paragraph _layoutTemplate;
             }
         }
 
-        public TextAlign? textAlign {
+        public TextAlign textAlign {
             get { return _textAlign; }
             set {
                 if (_textAlign == value) {

com.unity.uiwidgets/Runtime/rendering/editable.cs

         
         bool _onlyWhitespace(TextRange range) {
             for (int i = range.start; i < range.end; i++) {
-                int codeUnit = text.codeUnitAt(i).Value;
+                int codeUnit = text.codeUnitAt(i) ?? 0;
                 if (!EditableUtils._isWhitespace(codeUnit)) {
                     return false;
                 }
             }
         }
 
-        public TextAlign? textAlign {
+        public TextAlign textAlign {
             get { return _textPainter.textAlign; }
             set {
                 if (_textPainter.textAlign == value) {

com.unity.uiwidgets/Runtime/rendering/flex.cs

         }
 
         public float _getIntrinsicSize(
-            Axis? sizingDirection = null,
-            float? extent = null,
-            _ChildSizingFunction childSize = null
+            Axis sizingDirection,
+            float extent,
+            _ChildSizingFunction childSize
         ) {
             if (_direction == sizingDirection) {
                 float totalFlex = 0.0f;
                     int flex = _getFlex(child);
                     totalFlex += flex;
                     if (flex > 0) {
-                        float flexFraction = childSize(child, extent.Value) / _getFlex(child);
+                        float flexFraction = childSize(child, extent) / _getFlex(child);
-                        inflexibleSpace += childSize(child, extent.Value);
+                        inflexibleSpace += childSize(child, extent);
                     }
 
                     var childParentData = (FlexParentData) child.parentData;
                     child = childParentData.nextSibling;
                 }
 
-                float spacePerFlex = Mathf.Max(0.0f, ((availableMainSpace - inflexibleSpace) / totalFlex).Value);
+                float spacePerFlex = Mathf.Max(0.0f, ((availableMainSpace - inflexibleSpace) / totalFlex) ?? 0.0f);
 
                 child = firstChild;
                 while (child != null) {

com.unity.uiwidgets/Runtime/rendering/paragraph.cs

         }
 
         public TextAlign textAlign {
-            get { return _textPainter.textAlign.Value; }
+            get { return _textPainter.textAlign; }
             set {
                 if (_textPainter.textAlign == value) {
                     return;
                 placeholderDimensions[childIndex] = new PlaceholderDimensions(
                 size: new Size(child.getMaxIntrinsicWidth(height), height),
                 alignment: _placeholderSpans[childIndex].alignment,
-                baseline: _placeholderSpans[childIndex].baseline.Value,
+                baseline: _placeholderSpans[childIndex].baseline,
                 baselineOffset:0.0f
             );
             child = childAfter(child);
                 placeholderDimensions[childIndex] = new PlaceholderDimensions(
                     size: new Size(intrinsicWidth, intrinsicHeight),
                     alignment: _placeholderSpans[childIndex].alignment,
-                    baseline: _placeholderSpans[childIndex].baseline.Value,
+                    baseline: _placeholderSpans[childIndex].baseline,
                     baselineOffset:0.0f
                 );
                 child = childAfter(child);
                 placeholderDimensions[childIndex] = new PlaceholderDimensions(
                     size: new Size(intrinsicWidth, intrinsicHeight),
                     alignment: _placeholderSpans[childIndex].alignment,
-                    baseline: _placeholderSpans[childIndex].baseline.Value,
+                    baseline: _placeholderSpans[childIndex].baseline,
                     baselineOffset:0.0f
                 );
                 child = childAfter(child);
                 switch (_placeholderSpans[childIndex].alignment) {
                     case ui.PlaceholderAlignment.baseline: {
                         baselineOffset = child.getDistanceToBaseline(
-                        _placeholderSpans[childIndex].baseline.Value
-                        ).Value;
+                        _placeholderSpans[childIndex].baseline
+                        ) ?? 0.0f;
                         break;
                     }
                     default: {
                 _placeholderDimensions[childIndex] = new PlaceholderDimensions(
                     size: child.size,
                     alignment: _placeholderSpans[childIndex].alignment,
-                    baseline: _placeholderSpans[childIndex].baseline.Value,
+                    baseline: _placeholderSpans[childIndex].baseline,
                     baselineOffset: baselineOffset
                 );
                 child = childAfter(child);

com.unity.uiwidgets/Runtime/rendering/shifted_box.cs

                 child.layout(constraints.loosen(), parentUsesSize: true);
                 float? childBaseline = child.getDistanceToBaseline(baselineType);
                 float actualBaseline = baseline;
-                float top = actualBaseline - childBaseline.Value;
+                float top = actualBaseline - (childBaseline ?? 0.0f);
                 var childParentData = (BoxParentData) child.parentData;
                 childParentData.offset = new Offset(0.0f, top);
                 Size childSize = child.size;

com.unity.uiwidgets/Runtime/rendering/sliver_fixed_extent_list.cs

 
             if (firstChild != null) {
                 int leadingGarbage = _calculateLeadingGarbage(firstIndex);
-                int trailingGarbage = _calculateTrailingGarbage(targetLastIndex.Value);
+                int trailingGarbage = targetLastIndex == null ? 0: _calculateTrailingGarbage(targetLastIndex.Value);
                 collectGarbage(leadingGarbage, trailingGarbage);
             }
             else {

com.unity.uiwidgets/Runtime/widgets/editable_text.cs

         public readonly StrutStyle strutStyle;
         public readonly Color selectionColor;
         public readonly float textScaleFactor;
-        public readonly TextAlign? textAlign;
+        public readonly TextAlign textAlign;
         public readonly TextDirection? textDirection;
         public readonly bool obscureText;
         public readonly TextWidthBasis textWidthBasis;
             StrutStyle strutStyle = null,
             Color selectionColor = null,
             float textScaleFactor = 1.0f,
-            TextAlign? textAlign = null,
+            TextAlign textAlign = TextAlign.start,
             TextDirection? textDirection = null,
             Locale locale = null,
             bool obscureText = false,
                 strutStyle: strutStyle,
                 selectionColor: selectionColor,
                 textScaleFactor: textScaleFactor,
-                textAlign: textAlign ?? TextAlign.start,
+                textAlign: textAlign,
                 textDirection: textDirection,
                 locale: locale ?? Localizations.localeOf(context, nullOk: true),
                 selection: value.selection,

com.unity.uiwidgets/Runtime/widgets/widget_span.cs

     class WidgetSpan : PlaceholderSpan,IEquatable<WidgetSpan> {
         public WidgetSpan(
             Widget child ,
-            TextBaseline? baseline = null,
+            TextBaseline baseline = TextBaseline.alphabetic,
             TextStyle style = null,
             PlaceholderAlignment alignment = PlaceholderAlignment.bottom
         ) : base(