Add additional check for GUI shader compilation

src/gui/gui.cpp

@@ -301,6 +301,10 @@ void GUI::initialize() {
 void GUI::initializeGL() {
 	_program = ghoul::opengl::ProgramObject::Build("GUI",
 		"${SHADERS}/gui_vs.glsl", "${SHADERS}/gui_fs.glsl");
+	if (!_program) {
+		LERROR("Could not load program object for GUI");
+		return;
+	}
 		
     positionLocation = glGetAttribLocation(*_program, "in_position");
     uvLocation = glGetAttribLocation(*_program, "in_uv");