Proper code

[gib]

I'm sure I'm not using the correct terminology. I will explain how mainwindow.ui was created.
For testing, I started with another simple Qt program, one that doesn't use Qwt (this program works fine.) In Qt Designer I clicked-and-dragged a QMdiArea widget onto the central widget. This is why I said I "added" it to the central widget. I do everything in Designer, and do not call setCentralWidget(). I will attach mainwindow.ui. You will see that it contains several things that are unrelated to what I am trying to do - these are from the other program. You can ignore everything except mdiArea.
Thanks again for trying to help me! Unfortunately I am not a good C++ programmer, although I usually get things to work eventually.