Rewrite ForceVariableDialog. It's not based on wx.TextEntryDialog
Don't customize standard wx.TextEntryDialog on the fly, implement your
own dialog for that with expected behavior and without any hacks.
<xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>