[GREENFOOT-618] 'Convert Class To Java' can cause exception

The 'Convert Class To Java' menu item (from the main menu) can cause an exception when called on a class that is already Java.

Normally, the menu item should be greyed out if the class is Java. The following sequence leaves the item enabled for a Java class:

select a Stride class
select 'Convert to Java'. Confirm. (Conversion happens, class remains selected; menu item remains enabled.)
select 'Convert to Java' again.

Possible fix: either disable menu item after conversion, or de-select class after conversion.

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