do not allow time window with no mode set on session while loading mode

lib/ace/edit_session.js

@@ -772,6 +772,10 @@ var EditSession = function(text, mode) {
         if (module)
             return done(module);
 
+        // set mode to text until loading is finished
+        if (!this.$mode)
+            this.$setModePlaceholder();
+
         fetch(function() {
             require([mode], done);
         });
@@ -799,6 +803,27 @@ var EditSession = function(text, mode) {
         }
     };
 
+    this.$setModePlaceholder = function() {
+        var textModeId = "ace/mode/text"
+        if (!this.$modes[textModeId])
+            this.$modes[textModeId] = new TextMode();
+
+        var tokenizer = this.$modes[textModeId].getTokenizer();
+
+        if (!this.bgTokenizer) {
+            this.bgTokenizer = new BackgroundTokenizer(tokenizer);
+            var _self = this;
+            this.bgTokenizer.addEventListener("update", function(e) {
+                _self._emit("tokenizerUpdate", e);
+            });
+        } else {
+            this.bgTokenizer.setTokenizer(tokenizer);
+        }
+
+        this.tokenRe = this.$modes[textModeId].tokenRe;
+        this.nonTokenRe = this.$modes[textModeId].nonTokenRe;
+    };
+
     /**
     * EditSession.setMode(mode) 
     * - mode (TextMode): Set a new text mode
@@ -809,6 +834,7 @@ var EditSession = function(text, mode) {
     this.$mode = null;
     this.$modeId = null;
     this.setMode = function(mode) {
+        mode = mode || "ace/mode/text";
         // load on demand
         if (typeof mode === "string") {
             if (this.$modeId == mode)
@@ -823,12 +849,6 @@ var EditSession = function(text, mode) {
                 _self.setMode(module);
             });
             return;
-        } else if (mode == null) {
-            mode = "ace/mode/text"
-            this.$modeId = mode;
-            this.$modes[mode] = this.$modes[mode] || (new TextMode());
-            this.setMode(this.$modes[mode]);
-            return;
         }
 
         if (this.$mode === mode) return;