Second windows opens: GDI+ Window

When I open some sktechup files, a second window seems to open in the taskbar menu → her name is “GDI+ window (SktechUp.exe)”. Do you know what this is due to? a particular element in the file or its recording?
Thank’s a lot for your help.

Do you have a render extension install, they are the usual reason.

Yes, I have Twilight Render.
But this “windows” doesn’t appear on each file I open.
Maybe I have something to disable on this file?