Fix widgets not unsubscribing if type doesn't have a widget implement…

…ation (#43)

If you drag a table containing a widget that doesn't have a widget
implementation, it will never unsubscribe

This has the widget model be unsubscribed and disposed before returning
null in the tree row's `toWidgetContainerModel()` method

lib/widgets/network_tree/networktables_tree_row.dart

@@ -241,6 +241,10 @@ class NetworkTableTreeRow {
     NTWidget? widget = NTWidgetBuilder.buildNTWidgetFromModel(primary);
 
     if (widget == null) {
+      primary.unSubscribe();
+      primary.disposeWidget(deleting: true);
+      primary.forceDispose();
+
       return null;
     }