Add View::isNull()

ViewWrappers don't have ownership of the gui element.
The gui element might have been deleted, leaving us with a
stale wrapper

For impl simply use a QPointer in the QtWidgets and QtQuick
backends

src/qtquick/views/ViewWrapper_qtquick.cpp

@@ -168,7 +168,7 @@ std::shared_ptr<ViewWrapper> ViewWrapper_qtquick::parentView() const
 
 HANDLE ViewWrapper_qtquick::handle() const
 {
-    return reinterpret_cast<HANDLE>(m_item);
+    return reinterpret_cast<HANDLE>(m_item.data());
 }
 
 void ViewWrapper_qtquick::grabMouse()
@@ -192,3 +192,8 @@ QString ViewWrapper_qtquick::objectName() const
 {
     return {};
 }
+
+bool ViewWrapper_qtquick::isNull() const
+{
+    return m_item.data() == nullptr;
+}