Well I found the answer...
all I did was to eliminate the minimum size
self.subwindow.setMinimumSize(400, 200)
create a function for closing subwindows:
def closeIt(self):
self.vtn.mdiApps.removeSubWindow(self.subwindow)
and modified each function, for example with the OpenCalc function:
def OpenCalc(self):
a = self.calc.height() + 20
b = self.calc.width()
if self.vtn.mdiApps.activeSubWindow() is None:
self.subwindow.setMinimumSize(b, a)
self.subwindow.setWidget(self.calc)
self.vtn.mdiApps.addSubWindow(self.subwindow)
self.subwindow.show()
self.subwindow.widget().show()
after that everything is in working order.