I dont know what is causing the problem.
I checked it in Microsoft Edge. It seems to be working fine. Check the screenshot.
Do one thing.
1. After clicking 'Save As'-> Send me the console message. Right Click in chrome and select 'Inspect'. Goto 'Console' tab and send me the screenshot of that tab.
2. Clear the mindmap app from your browser. Select Chrome Menu->More Tools->Clear Browsing Data... Then clear all time data. Then try reloading the mindmap webapp.
3. Also, you can try using mindmap from a browser where you have never the use the app before. Use a fresh browser.