[GREENFOOT-781] Controls enabled when no world, then disabled if clicked

Load up a scenario where there is no default world (in my test case, there is a world, but it has not been stored as the last instantiated one). The Act/Run/Reset buttons are all enabled, which they shouldn't be (although that's not a terrible problem if they just do nothing). But clicking Run or Reset gets into a state where all the buttons are disabled, as in the bug GREENFOOT-780. If you clicked Run, making a new World manually does fix the issue, but if you clicked Reset, the buttons remain disabled and they cannot be recovered.

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Issue metadata

• Issue type: Task
• Priority: Medium