Fix crash in auto hide tab (#552)

- Crash is caused by dragging an autohide tab out when it is an icon only.
- The crash can actually happen with non icon only tabs, but is masked by the orientation check.
- Soln: Check if auto hide container exists first on mouse finish dragging event.

Author: SyarifFakhri

src/AutoHideTab.cpp

@@ -478,7 +478,7 @@ void CAutoHideTab::mouseReleaseEvent(QMouseEvent* ev)
 		case DraggingFloatingWidget:
 			 ev->accept();
 			 d->FloatingWidget->finishDragging();
-			 if (d->DockWidget->isAutoHide() && d->DragStartOrientation != orientation())
+			 if (d->DockWidget->autoHideDockContainer() && d->DragStartOrientation != orientation())
 			 {
 				 d->DockWidget->autoHideDockContainer()->resetToInitialDockWidgetSize();
 			 }