Qt Threading Issues in Linux
OpenGL, Qt rendering and multithreading don't mix well. In particular a OpenGL context can be active in only one thread at a time. Now if the context is shared among multiple widgets (note that this different from sharing objects between contexts, I'm talking about a single context that's used for multiple windows/widgets which is legitimate) and those widgets render from different threads you're going to get into a lot of issues.
Usually the best course of action when it comes to OpenGL and multithreading is, not doing it. Use multiple threads, yes, but use them for everything that's not related to OpenGL or any kind of graphics output. Keep all graphics operations to a single thread to avoid major issues.