fix task order

Samples/UIWidgetsSamples_2019_4/Assets/TextTest.unity

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com.unity.uiwidgets/Runtime/engine2/UIWidgetsPanel.cs

         }
 
         protected override void OnRectTransformDimensionsChange() {
-            if (_ptr != IntPtr.Zero) {
+            if (_ptr != IntPtr.Zero  && _renderTexture) {
                 if (_recreateRenderTexture(_currentWidth, _currentHeight, _currentDevicePixelRatio)) {
                     UIWidgetsPanel_onRenderTexture(_ptr,
                         _renderTexture.GetNativeTexturePtr(),

engine/src/shell/platform/unity/win32_task_runner.cc

   for (const auto& observer : task_observers_) {
     observer.second();
   }
-	
+
   // Fire expired tasks.
   {
     // Flushing tasks here without holing onto the task queue mutex.
 
 Win32TaskRunner::TaskTimePoint Win32TaskRunner::TimePointFromUIWidgetsTime(
     uint64_t uiwidgets_target_time_nanos) {
+  const auto fml_now = fml::TimePoint::Now().ToEpochDelta().ToNanoseconds();
+  if (uiwidgets_target_time_nanos <= fml_now) {
+    return {};
+  }
+  const auto uiwidgets_duration = uiwidgets_target_time_nanos - fml_now;
-  const auto uiwidgets_duration =
-      uiwidgets_target_time_nanos -
-      fml::TimePoint::Now().ToEpochDelta().ToNanoseconds();
   return now + std::chrono::nanoseconds(uiwidgets_duration);
 }