Bug 389334 - Helper app no longer prefilled. r=mano

toolkit/mozapps/downloads/src/nsHelperAppDlg.js.in

@@ -677,7 +677,13 @@ nsUnknownContentTypeDialog.prototype = {
       }
     
       // Fill in helper app info, if there is any.
-      this.chosenApp = this.mLauncher.MIMEInfo.preferredApplicationHandler;
+      try {
+        this.chosenApp =
+          this.mLauncher.MIMEInfo.preferredApplicationHandler
+              .QueryInterface(Components.interfaces.nsILocalHandlerApp);
+      } catch (e) {
+        this.chosenApp = null;
+      }
       // Initialize "default application" field.
       this.initDefaultApp();