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/View.cpp

@@ -258,3 +258,8 @@ bool View::equals(const std::shared_ptr<ViewWrapper> &other) const
 {
     return other && handle() == other->handle();
 }
+
+bool View::isNull() const
+{
+    return false;
+}

src/View.h

@@ -93,6 +93,12 @@ public:
     /// as long as it uniquely identifies the GUI element. KDDW backend only uses it for comparison purposes
     virtual HANDLE handle() const = 0;
 
+    /// @brief Returns whether the gui item represented by this view was already deleted
+    /// Usually false, as KDDW internal gui elements inherit View, and nobody will access them after destruction.
+    /// However, ViewWrapper derived classes, wrap an existing gui element, which might get deleted.
+    /// Override isNull() in our ViewWrapper subclasses and return true if the wrapped gui element was already deleted
+    virtual bool isNull() const;
+
     /// @brief Called by the layouting engine
     /// Override it in case your widget needs to know where it is in the layout. Usually only needed by Frame.s
     virtual void setLayoutItem(Layouting::Item *) {};

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;
+}

src/qtquick/views/ViewWrapper_qtquick.h

@@ -14,6 +14,7 @@
 #include "ViewWrapper.h"
 
 #include <QQuickItem>
+#include <QPointer>
 
 namespace KDDockWidgets::Views {
 
@@ -50,9 +51,10 @@ public:
     QScreen *screen() const override;
     void setFocus(Qt::FocusReason) override;
     QString objectName() const override;
+    bool isNull() const override;
 
 private:
-    QQuickItem *const m_item;
+    QPointer<QQuickItem> m_item;
 };
 
 }

src/qtwidgets/views/ViewWrapper_qtwidgets.cpp

@@ -243,7 +243,7 @@ std::shared_ptr<ViewWrapper> ViewWrapper_qtwidgets::parentView() const
 
 HANDLE ViewWrapper_qtwidgets::handle() const
 {
-    return reinterpret_cast<HANDLE>(m_widget);
+    return reinterpret_cast<HANDLE>(m_widget.data());
 }
 
 void ViewWrapper_qtwidgets::grabMouse()
@@ -270,3 +270,8 @@ QString ViewWrapper_qtwidgets::objectName() const
 {
     return m_widget->QWidget::objectName();
 }
+
+bool ViewWrapper_qtwidgets::isNull() const
+{
+    return m_widget.data() == nullptr;
+}

src/qtwidgets/views/ViewWrapper_qtwidgets.h

@@ -14,6 +14,7 @@
 #include "ViewWrapper.h"
 
 #include <QWidget>
+#include <QPointer>
 
 namespace KDDockWidgets::Views {
 
@@ -50,9 +51,10 @@ public:
     QScreen *screen() const override;
     void setFocus(Qt::FocusReason) override;
     QString objectName() const override;
+    bool isNull() const override;
 
 private:
-    QWidget *const m_widget;
+    QPointer<QWidget> m_widget;
 };
 
 }