Bug 1597478 - follow up: add the null check for `mPresContext`. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D61416

--HG--
extra : moz-landing-system : lando

layout/base/nsDocumentViewer.cpp

@@ -3871,6 +3871,9 @@ bool nsDocumentViewer::ShouldAttachToTopLevel() {
 
 #if defined(XP_WIN) || defined(MOZ_WIDGET_GTK) || \
     defined(MOZ_WIDGET_ANDROID) || defined(MOZ_WIDGET_UIKIT)
+  if (!mPresContext) {
+    return false;
+  }
 
   // On windows, in the parent process we also attach, but just to
   // chrome items