Protect against the group being destroyed before the tabIndex code could be run.

PiperOrigin-RevId: 185259776

Author: TedSander

lib/material_radio/material_radio_group.dart

@@ -166,6 +166,7 @@ class MaterialRadioGroupComponent
     // Since this is updating children that were already dirty-checked,
     // need to delay this change until next angular cycle.
     _ngZone.onEventDone.first.then((_) {
+      if (_children == null) return; // Component was destoryed.
       // Disable everything first.
       for (var child in _children) {
         child.tabbable = false;