bug fix

Runtime/ui/txt/paragraph.cs

                     (!ReferenceEquals(item.Value, null) && item.Value.Equals(this.root.Value)));
         }
 
-        public KeyValuePair<TKey, TValue> First() {
+        public KeyValuePair<TKey, TValue>? First() {
-                throw new NullReferenceException("The root of this tree is null!");
+                return null;
             }
 
             while (t.LeftChild != null) {
             return new KeyValuePair<TKey, TValue>(t.Key, t.Value);
         }
 
-        public KeyValuePair<TKey, TValue> Last() {
+        public KeyValuePair<TKey, TValue>? Last() {
-                throw new NullReferenceException("The root of this tree is null!");
+                return null;
             }
 
             while (t.RightChild != null) {

Runtime/widgets/list_wheel_scroll_view.cs

                 return;
             }
 
-            int firstIndex = this._childElements.First().Key;
-            int lastIndex = this._childElements.Last().Key;
+            int firstIndex = this._childElements.First()?.Key ?? 0;
+            int lastIndex = this._childElements.Last()?.Key ?? 0;
 
             for (int index = firstIndex; index <= lastIndex; ++index) {
                 Element newChild = this.updateChild(this._childElements[index], this.retrieveWidget(index), index);

Runtime/widgets/sliver.cs

                 // processElement may modify the Map - need to do a .toList() here.
                 this._childElements.Keys.ToList().ForEach(action: processElement);
                 if (this._didUnderflow) {
-                    var lastKey = this._childElements?.Last().Key ?? -1;
+                    var lastKey = this._childElements?.Last()?.Key ?? -1;
                     processElement(lastKey + 1);
                 }
             }