textspan equal

Runtime/painting/text_painter.cs

         public TextSpan text {
             get { return this._text; }
             set {
-                if ((this._text == null && value == null) || this.text.Equals(value)) {
+                if ((this._text == null && value == null) || (this._text != null &&  this.text.Equals(value))) {
                     return;
                 }