uitest: we also need a SolarMutex when deleting the wrapper object

The wrapper has references to vcl objects and the destructor might have
the last reference to a vcl::Window causing a delete. Avoid potential
problems (I have seen one deadlock in the chart wizard code) by taking
the SolarMutex.

Change-Id: I8afcf79842b2f8e8f822b975629d5c8e18a86163

vcl/source/uitest/uno/uiobject_uno.cxx

@@ -20,6 +20,8 @@ UIObjectUnoObj::UIObjectUnoObj(std::unique_ptr<UIObject> pObj):
 
 UIObjectUnoObj::~UIObjectUnoObj()
 {
+    SolarMutexGuard aGuard;
+    mpObj.reset();
 }
 
 css::uno::Reference<css::ui::test::XUIObject> SAL_CALL UIObjectUnoObj::getChild(const OUString& rID)