--- a/devtools/client/framework/source-map-url-service.js
+++ b/devtools/client/framework/source-map-url-service.js
@@ -68,16 +68,21 @@ SourceMapURLService.prototype.destroy = 
   Services.prefs.removeObserver(SOURCE_MAP_PREF, this._onPrefChanged);
   this._target = this._urls = this._subscriptions = null;
 };
 
 /**
  * A helper function that is called when a new source is available.
  */
 SourceMapURLService.prototype._onSourceUpdated = function (_, sourceEvent) {
+  // Maybe we were shut down while waiting.
+  if (!this._urls) {
+    return;
+  }
+
   let { source } = sourceEvent;
   let { generatedUrl, url, actor: id, sourceMapURL } = source;
 
   // |generatedUrl| comes from the actor and is extracted from the
   // source code by SpiderMonkey.
   let seenUrl = generatedUrl || url;
   this._urls.set(seenUrl, { id, url: seenUrl, sourceMapURL });
 };