bug when opening a tabbed HTML viewer on v3.1 that didn't see on v3.0

[davidpsharp]

I'm seeing unusual breaking behaviour on cheerpj v3.1 that I didn't see on v3.0.

If you go to https://davidsharp.com/baby/online/index.html (v3.1), let the application load and then from the Help menu tick 'View Reference Manual Tab' it should add a Tabbed panel and render a simple HTML file over most of the display but instead I get the following on Chrome (Mac) developer console, the menu locks up and doesn't close and the application appears to crash:

Exception in thread "AWT-EventQueue-0" cheerpOS.js:1944 java.lang.ClassCastException cheerpOS.js:1944

cheerpOS.js:1944 Exception in thread "AWT-EventQueue-0" cheerpOS.js:1944 java.lang.ClassCastException cheerpOS.js:1944

If you run the same JAR on cheerpj v3.0 it works fine as per: https://davidsharp.com/baby/online/index30.html

Source code for that that menu and that JAR as of that commit is: https://github.com/davidpsharp/baby/blob/6e1f6e7b54db109ebd08057f4d16a6e07038076a/src/main/java/com/ccs/baby/menu/HelpMenu.java