why...when the auto fill allows me to select .render as a method of MainWindow?
Do you have a member variable of your MainWindow class named "MainWindow"? If not, do you know C++? If the answers are "No" and "Yes", then you can probably figure out why line 271 won't compile.

And learn to use [code ] & [/code ] tags to wrap the code lines when you post source code. (With no spaces after the word "code" in the square brackets).