Guard code for context.mode

In case this is somehow called before initialization is complete
It happens rarely when restoring the initial changes and deferring rendering

modules/ui/modes.js

@@ -24,7 +24,8 @@ export function uiModes(context) {
 
 
     function editable() {
-        return context.editable() && context.mode().id !== 'save';
+        var mode = context.mode();
+        return context.editable() && mode && mode.id !== 'save';
     }