Handle the case where no mode is loaded a little more gracefully

Author: marijnh

lib/codemirror.js

@@ -1439,14 +1439,14 @@ var CodeMirror = (function() {
       spec = mimeModes[spec];
     if (typeof spec == "string")
       var mname = spec, config = {};
-    else
+    else if (spec != null)
       var mname = spec.name, config = spec;
     var mfactory = modes[mname];
     if (!mfactory) {
       if (window.console) console.warn("No mode " + mname + " found, falling back to plain text.");
       return CodeMirror.getMode(options, "text/plain");
     }
-    return mfactory(options, config);
+    return mfactory(options, config || {});
   }
   CodeMirror.listModes = function() {
     var list = [];