bugfix: "crashes when quit with 'close window' button"

[Harald Fernengel]

Hello Heinz,

> Hello kdevelop-team,
> I'm really fond of KDevelop and Gideon!
>
> I hunted the bug " crashes when quit with 'close window' button" and made a
> tiny patch:
>
> The bugfix is in the file doctreeviewpart.cpp. It's trivial, just two lines
> of code have to be deleted (see doctreeviewpart.cpp.patch).
> The buggy code accessed an already deleted object though
> TopLevel-getInstance()->...
>
> I changed the class TopLevel so it will assert if someone tries a similar
> thing in the future. This may help finding a bug quicker (see
> toplevel.patch).

I just commited your two patches, but without the assert, because this will 
trigger on changing mdi modes on the fly (I know, it is disabled at the 
moment).

Thanks for finding the bugs, it also helped me to fix the diff part, so gideon 
exits cleanly now :)

Best regards,
Harry