error when plotting with graphics_toolkit fltk : "... octave-cli-3.8.2[30626] <Error>: CGContextFillRects: invalid context 0x0...."

[sts]

I have just installed octave 3.8.2 on osx 10.9.5 via macport.

When I try to plot with graphics_toolkit fltk I see the messages:

octave:1> graphics_toolkit('fltk')
octave:2> plot(1)
May 13 18:16:39 sts-MacBook-Pro.local octave-cli-3.8.2[30626] <Error>: 
CGContextFillRects: invalid context 0x0. This is a serious error. This 
application, or a library it uses, is using an invalid context  and is 
thereby contributing to an overall degradation of system stability and 
reliability. This notice is a courtesy: please fix this problem. It will 
become a fatal error in an upcoming update.
May 13 18:16:39 sts-MacBook-Pro.local octave-cli-3.8.2[30626] <Error>: 
CGContextMoveToPoint: invalid context 0x0. This is a serious error. This 
application, or a library it uses, is using an invalid context  and is 
thereby contributing to an overall degradation of system stability and 
reliability. This notice is a courtesy: please fix this problem. It will 
become a fatal error in an upcoming update.
May 13 18:16:39 sts-MacBook-Pro.local octave-cli-3.8.2[30626] <Error>: 
CGContextAddLineToPoint: invalid context 0x0. This is a serious error. 
This application, or a library it uses, is using an invalid context  and 
is thereby contributing to an overall degradation of system stability 
and reliability. This notice is a courtesy: please fix this problem. It 
will become a fatal error in an upcoming update.
...
...
...
May 13 18:16:40 sts-MacBook-Pro.local octave-cli-3.8.2[30626] <Error>: 
CGContextAddLineToPoint: invalid context 0x0. This is a serious error. 
This application, or a library it uses, is using an invalid context  and 
is thereby contributing to an overall degradation of system stability 
and reliability. This notice is a courtesy: please fix this problem. It 
will become a fatal error in an upcoming update.
May 13 18:16:40 sts-MacBook-Pro.local octave-cli-3.8.2[30626] <Error>: 
CGContextDrawPath: invalid context 0x0. This is a serious error. This 
application, or a library it uses, is using an invalid context  and is 
thereby contributing to an overall degradation of system stability and 
reliability. This notice is a courtesy: please fix this problem. It will 
become a fatal error in an upcoming update.
May 13 18:16:40 sts-MacBook-Pro.local octave-cli-3.8.2[30626] <Error>: 
CGContextFillRects: invalid context 0x0. This is a serious error. This 
application, or a library it uses, is using an invalid context  and is 
thereby contributing to an overall degradation of system stability and 
reliability. This notice is a courtesy: please fix this problem. It will 
become a fatal error in an upcoming update.
Fontconfig warning: ignoring UTF-8: not a valid region tag
octave:3>

here I have reported only the first three and the last three messages, 
but there are a few hundreds of other similar messages in between. After 
that the plot looks ok. When I use gnuplot, instead I get

octave:1> graphics_toolkit('gnuplot')
octave:2> plot(1)
Fontconfig warning: ignoring UTF-8: not a valid region tag
octave:3>

And the plot looks fine also in this case.
I have no idea what is the meaning of these messages. Can anyone help me 
to understand where the problem could be?

Thanks
sts