Some clang-tidy -use-modernize-nullptr

Change-Id: I1bed5e85a5b7948d08502a72a10f80baa075c204
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/designer/formwindowfile.cpp

@@ -286,7 +286,7 @@ void FormWindowFile::slotFormWindowRemoved(QDesignerFormWindowInterface *w)
     // as calls to isDirty() are triggered at arbitrary times
     // while building.
     if (w == m_formWindow)
-        m_formWindow = 0;
+        m_formWindow = nullptr;
 }
 
 } // namespace Internal