Merge>87: Fix remaining crashes where main frame is likely null

- Add null check for MainFrame() similar to elsewhere in
  VisualViewport::UpdatePaintPropertyNodesIfNeeded()

(cherry picked from commit b79ce46d6ad3bbfc1133396fd00e2da6a77edc1b)

Bug: 1134597
Change-Id: Iaaf6d91788051c650f8ed270787cf47782649053
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468737
Auto-Submit: Aaron Leventhal <[email protected]>
Reviewed-by: Chris Harrelson <[email protected]>
Commit-Queue: Chris Harrelson <[email protected]>
Commit-Queue: Aaron Leventhal <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#816814}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488004
Reviewed-by: Aaron Leventhal <[email protected]>
Cr-Commit-Position: refs/branch-heads/4280@{#548}
Cr-Branched-From: ea420fb963f9658c9969b6513c56b8f47efa1a2a-refs/heads/master@{#812852}
diff --git a/third_party/blink/renderer/core/frame/visual_viewport.cc b/third_party/blink/renderer/core/frame/visual_viewport.cc
index d47860c..60c9b30d 100644
--- a/third_party/blink/renderer/core/frame/visual_viewport.cc
+++ b/third_party/blink/renderer/core/frame/visual_viewport.cc
@@ -325,7 +325,7 @@
   parent_property_tree_state_ =
       PropertyTreeStateOrAlias(*transform_parent, *clip_parent, *effect_parent);
 
-  if (change == PaintPropertyChangeType::kNodeAddedOrRemoved)
+  if (change == PaintPropertyChangeType::kNodeAddedOrRemoved && MainFrame())
     MainFrame()->View()->SetVisualViewportNeedsRepaint();
 
   return change;