fix #1893

platform/firefox/frameModule.js

@@ -231,6 +231,14 @@ var contentObserver = {
             context = (context.ownerDocument || context).defaultView;
         }
 
+        // https://github.com/gorhill/uBlock/issues/1893
+        // I don't know why this happens. I observed that when it occurred, the
+        // resource was not seen by the HTTP observer, as if it was a spurious
+        // call to shouldLoad().
+        if ( !context ) {
+            return this.ACCEPT;
+        }
+
         // The context for the toolbar popup is an iframe element here,
         // so check context.top instead of context
         if ( !context.top || !context.location ) {

platform/firefox/vapi-client.js

@@ -23,14 +23,14 @@
    addMessageListener, removeMessageListener, sendAsyncMessage, outerShutdown
  */
 
+'use strict';
+
 // For non background pages
 
 /******************************************************************************/
 
 (function(self) {
 
-'use strict';
-
 // https://github.com/chrisaljoudi/uBlock/issues/464
 if ( document instanceof HTMLDocument === false ) {
     // https://github.com/chrisaljoudi/uBlock/issues/1528

src/js/contentscript.js

@@ -1500,16 +1500,17 @@ vAPI.domSurveyor = (function() {
 /******************************************************************************/
 
 vAPI.domIsLoaded = function(ev) {
+    // This can happen on Firefox. For instance:
+    // https://github.com/gorhill/uBlock/issues/1893
+    if ( window.location === null ) {
+        return;
+    }
+
     if ( ev instanceof Event ) {
         document.removeEventListener('DOMContentLoaded', vAPI.domIsLoaded);
     }
     vAPI.domIsLoaded = null;
 
-    // I've seen this happens on Firefox
-    if ( window.location === null ) {
-        return;
-    }
-
     vAPI.executionCost.start();
 
     vAPI.domWatcher.start();