Bug 1349223 - do not assert if a document is unattached from tree when seding the events, r=jamie

accessible/windows/msaa/AccessibleWrap.cpp

@@ -1338,8 +1338,13 @@ AccessibleWrap::GetHWNDFor(Accessible* aAccessible)
     // their tab is within.  Popups are always in the parent process, and so
     // never proxied, which means this is basically correct.
     Accessible* outerDoc = proxy->OuterDocOfRemoteBrowser();
-    NS_ASSERTION(outerDoc, "no outer doc for accessible remote tab!");
     if (!outerDoc) {
+      // In some cases, the outer document accessible may be unattached from its
+      // document at this point, if it is scheduled for removal. Do not assert
+      // in such case. An example: putting aria-hidden="true" on HTML:iframe
+      // element will destroy iframe's document asynchroniously, but
+      // the document may be a target of selection events until then, and thus
+      // it may attempt to deliever these events to MSAA clients.
       return nullptr;
     }