[Lazarus] how to find this error?

[FreeMan]

fpc r31177 lazarus r49465 qt x64 osx
my project work in osx qt64, win32 i386 qt is workas well, but in win64 
qt64 create "access violation" exception on "myform.Show;"
on form has a "TStatusBar"
I := TStatusBar.Canvas.TextWidth(TStatusBar.Panels[0].Text);
this code has create exception in win64 too.
when I wrote this message, 
http://forum.lazarus.freepascal.org/index.php/topic,27706.0.html same 
form and same my code worked, I mean my code not changed but not working 
now. I remark this line and re try run under win64, but still raise 
exception when use myForm.Show;
I can not trace this exception, I added onShow event but not trigger. 
raise exception and terminate application.
whats yours suggestion? TStatusBar bar has a bug ?? or what can be problem ?
Thank you