Bug 1677934 - Check for null when getting leaf for text marker. r=morgan, a=jcristau

This doesn't happen often. I can't find a reproducable case to test with. This happens with rapidly mutating content and is the result of some kind of race.

Differential Revision: https://phabricator.services.mozilla.com/D100174

accessible/mac/MOXTextMarkerDelegate.mm

@@ -291,7 +291,12 @@ static nsDataHashtable<nsUint64HashKey, MOXTextMarkerDelegate*> sDelegates;
     return nil;
   }
 
-  return GetNativeFromGeckoAccessible(geckoTextMarker.Leaf());
+  AccessibleOrProxy leaf = geckoTextMarker.Leaf();
+  if (leaf.IsNull()) {
+    return nil;
+  }
+
+  return GetNativeFromGeckoAccessible(leaf);
 }
 
 - (id)moxTextMarkerRangeForUIElement:(id)element {