Bug 1557409 - Part 1: Make sure nsIGlobalObject::PrincipalOrNull() returns null when called off the main thread; r=baku

Differential Revision: https://phabricator.services.mozilla.com/D34207

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

dom/base/nsIGlobalObject.cpp

@@ -37,6 +37,10 @@ nsIGlobalObject::~nsIGlobalObject() {
 }
 
 nsIPrincipal* nsIGlobalObject::PrincipalOrNull() {
+  if (!NS_IsMainThread()) {
+    return nullptr;
+  }
+
   JSObject* global = GetGlobalJSObjectPreserveColor();
   if (NS_WARN_IF(!global)) return nullptr;