closure/extensions: fix some nullability issues in extension error overlay.

chrome/browser/resources/extensions/extension_error_overlay.js

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * The type of the stack trace object. The definition is based on
  * extensions/browser/extension_error.cc:RuntimeError::ToValue().
  * @typedef {{columnNumber: number,
  *            functionName: string,
  *            lineNumber: number,
@@ skipping 224 matching lines @@
         // with that frame.
         frameNode.addEventListener('click', function(frame, frameNode, e) {
           this.setActiveFrame_(frameNode);
 
           // Request the file source with the section highlighted; this will
           // call ExtensionErrorOverlay.requestFileSourceResponse() when
           // completed, which in turn calls setCode().
           ExtensionErrorOverlay.requestFileSource(
               {extensionId: this.error_.extensionId,
                message: this.error_.message,
-               pathSuffix: getRelativeUrl(frame.url, this.extensionUrl_),
+               pathSuffix: getRelativeUrl(frame.url,
+                                          assert(this.extensionUrl_)),
                lineNumber: frame.lineNumber});
         }.bind(this, frame, frameNode));
 
         this.stackTrace_.appendChild(frameNode);
       }
 
       // Set the current stack frame to the first stack frame and show the
       // trace, if one exists. (We can't just check error.stackTrace, because
       // it's possible the trace was purely internal, and we don't show
       // internal frames.)
@@ skipping 279 matching lines @@
     var overlay = extensions.ExtensionErrorOverlay.getInstance();
     overlay.setCode(result);
     overlay.setVisible(true);
   };
 
   // Export
   return {
     ExtensionErrorOverlay: ExtensionErrorOverlay
   };
 });