Bug 1540839 - Don't set embedder element to null; r=nika

When resetting the BrowsingContext embedder element, only do so if the
FrameElement we're setting isn't already null.

Differential Revision: https://phabricator.services.mozilla.com/D30562

dom/ipc/BrowserParent.cpp

@@ -536,7 +536,7 @@ void BrowserParent::SetOwnerElement(Element* aElement) {
 
   // Set our BrowsingContext's embedder if we're not embedded within a
   // BrowserBridgeParent.
-  if (!GetBrowserBridgeParent() && mBrowsingContext) {
+  if (!GetBrowserBridgeParent() && mBrowsingContext && mFrameElement) {
     mBrowsingContext->SetEmbedderElement(mFrameElement);
   }