Bug 782877 - Intermittent browser_dbg_bfcache.js | uncaught JS exception - TypeError: element is undefined at debugger-controller.js:1275, r=rcampbell

browser/devtools/debugger/debugger-controller.js

@@ -1353,6 +1353,16 @@ SourceScripts.prototype = {
   _onLoadSourceFinished:
   function SS__onLoadSourceFinished(aScriptUrl, aSourceText, aContentType, aOptions) {
     let element = DebuggerView.Scripts.getScriptByLocation(aScriptUrl);
+
+    // Tab navigated before we got a chance to finish loading and displaying
+    // the source. The outcome is that the expected url is not present anymore
+    // in the scripts container, hence the original script object coming from
+    // the active thread no longer exists. There's really nothing that needs
+    // to be done in this case, nor something that can be currently avoided.
+    if (!element) {
+      return;
+    }
+
     let script = element.getUserData("sourceScript");
 
     script.loaded = true;