Bug 310139: Editing mime-type handlers fails with a missing handler, patch by Phil Ringnalda <bugzilla@philringnalda.com>, r=mscott

mail/components/preferences/downloadactions.js

@@ -298,6 +298,8 @@ var gDownloadActionsWindow = {
       catch (e) {
         customHandler = null;
       }      
+      if (customHandler && !customHandler.exists())
+        customHandler = null;
       var mimeType = this._getLiteralValue(type, "value");
       var typeInfo = this._mimeSvc.getFromTypeAndExtension(mimeType, null);