Porting GTK 1.2 ==> GTK 1.3

[zze-coframi balr001 <coframi balr001 rd francetelecom com>]

I developed an application under 1.2 and I want to port it to GTK 1.3
because I need to use the Linux FrameBuffer. 
I've an error : GRuntime Warning : Invalid cast from PangoXFont to <unknown>
I use a gtkrc file to define my own widget. How can I do this thing using
GTK 1.3 ??
Can someone tell me where that error comes from ??

thanks very much for help

Isabelle