button_theme GetHashCode fix

Runtime/material/button_theme.cs

                 hashCode = (hashCode * 397) ^ this.padding.GetHashCode();
                 hashCode = (hashCode * 397) ^ this.shape.GetHashCode();
                 hashCode = (hashCode * 397) ^ this.alignedDropdown.GetHashCode();
-                hashCode = (hashCode * 397) ^ this._buttonColor?.GetHashCode() ?? 1;
-                hashCode = (hashCode * 397) ^ this._disabledColor?.GetHashCode() ?? 1;
-                hashCode = (hashCode * 397) ^ this._highlightColor?.GetHashCode() ?? 1;
-                hashCode = (hashCode * 397) ^ this._splashColor?.GetHashCode() ?? 1;
+                hashCode = (hashCode * 397) ^ (this._buttonColor != null ? this._buttonColor.GetHashCode() : 0);
+                hashCode = (hashCode * 397) ^ (this._disabledColor != null ? this._disabledColor.GetHashCode() : 0);
+                hashCode = (hashCode * 397) ^ (this._highlightColor != null ? this._highlightColor.GetHashCode() : 0);
+                hashCode = (hashCode * 397) ^ (this._splashColor != null ? this._splashColor.GetHashCode() : 0);
                 hashCode = (hashCode * 397) ^ this.colorScheme.GetHashCode();
                 hashCode = (hashCode * 397) ^ this._materialTapTargetSize.GetHashCode();
                 return hashCode;