Fix potential crash found by the tests

Item::checkSanity() can be called via deleteLater when we don't
have a platform anymore

src/Platform.cpp

@@ -118,6 +118,7 @@ void Platform::tests_initPlatform(int &argc, char **argv, KDDockWidgets::Fronten
 void Platform::tests_deinitPlatform()
 {
     auto plat = Platform::instance();
+    plat->d->m_inDestruction = true;
 
     plat->tests_deinitPlatform_impl();
     delete plat;

src/private/Platform_p.h

@@ -28,5 +28,12 @@ public:
     /// to window activations and use a different style depending on activation state.
     KDBindings::Signal<std::shared_ptr<View>> windowActivated;
     KDBindings::Signal<std::shared_ptr<View>> windowDeactivated;
+
+    bool inDestruction() const
+    {
+        return m_inDestruction;
+    }
+
+    bool m_inDestruction = false;
 };
 }

src/private/multisplitter/Item.cpp

@@ -18,7 +18,8 @@
 #include "kdbindings/signal.h"
 #include "Window.h"
 #include "../View_p.h"
-
+#include "Platform.h"
+#include "../Platform_p.h"
 
 #include <QEvent>
 #include <QDebug>
@@ -1002,6 +1003,13 @@ bool ItemBoxContainer::checkSanity()
 {
     d->m_checkSanityScheduled = false;
 
+    auto plat = Platform::instance();
+    if (!plat || plat->d->inDestruction()) {
+        // checkSanity() can be called with deleteLater(), so check if we still
+        // have a platform
+        return true;
+    }
+
     if (!hostView()) {
         /// This is a dummy ItemBoxContainer, just return true
         return true;