Avoid erroring when removing null windows.

Change-Id: Id79031d5537cd8869e9195f2d944879b79a09f37
Reviewed-on: http://gerrit.dmdirc.com/3379
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>
Don't throw NPEs when rendering colours.

Change-Id: Icde4691b57af7a27b0eaaf82a18d1d7d036d7740
Reviewed-on: http://gerrit.dmdirc.com/3380
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>