[NSWindow _initBackendWindow] segmentation fault ...

Hello,

I've a segmentation fault on:
[NSWindow _initBackendWindow] at NSWindow.m:791

The line in the file is this one:
...
// Set window in new _gstate
DPSgsave(context);
...

With ddd, I found that "context" is nil (0x0) but it initialised with
that : "NSGraphicsContext *context = GSCurrentContext();".

So what are reason's that GSCurrentContext can return nil ?

To know, my last application call is a [NSImage lockFocus] with an
NSimage wich is init with "initWithSize" and wich respond "false" to
isValid (is it normal?). With the call to "lockFocus" I've the
segmentation fault describe above.

Thank's in advance for your help.

Christophe.

ps:
I use the "current release" of GNUStep.
I try both with art and xlib backend but I've the same bug.
My application is a port of a MAC/COCOA to gnustep. 
It have 2 threads.