Which way did you tried?

if u are setting widgets font with
Try like QFont font("font: Arial", 12, QFont:referDefault);
ui.someLabel->setFont(font);

make sure it is done after setupUi() (in constructor) because setupUi() creates widgets in your ui - calling anything like ui.widget->method() before widget is initialized will cause crash.

if this isn't a reason try give a bit more detailed description of crash you have.

would be easier to find a solution if u would write anything about the problem
so which method did u tried, when application crashes, where(maybe small part of code?), what debugger says etc.