Update selection_controls_demo.cs

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/material/selection_controls_demo.cs

                                 new Checkbox(
                                     value: checkboxValueC,
                                     tristate: true,
-                                    onChanged: (bool? value) => { setState(() => { checkboxValueC = value.Value; }); }
+                                    onChanged: (bool? value) => { setState(() => { checkboxValueC = value == null? false: value.Value; }); }
                                 )
                             }
                         ),