kcms/decoration: Fix crash when preview cannot be generated

m_bridge->createDecoration() can return nullptr. Most codepaths
already guard against that, but not all of them

BUG: 456531

src/kcms/decoration/declarative-plugin/previewitem.cpp

@@ -41,7 +41,9 @@ PreviewItem::PreviewItem(QQuickItem *parent)
 
 PreviewItem::~PreviewItem()
 {
-    m_decoration->deleteLater();
+    if (m_decoration) {
+        m_decoration->deleteLater();
+    }
     if (m_bridge) {
         m_bridge->unregisterPreviewItem(this);
     }
@@ -288,15 +290,17 @@ static QHoverEvent cloneEventWithPadding(QHoverEvent *event, int paddingLeft, in
         event->modifiers());
 }
 
-template <typename E>
+template<typename E>
 void PreviewItem::proxyPassEvent(E *event) const
 {
-    const auto &shadow = m_decoration->shadow();
-    if (shadow) {
-        E e = cloneEventWithPadding(event, shadow->paddingLeft(), shadow->paddingTop());
-        QCoreApplication::instance()->sendEvent(decoration(), &e);
-    } else {
-        QCoreApplication::instance()->sendEvent(decoration(), event);
+    if (m_decoration) {
+        const auto &shadow = m_decoration->shadow();
+        if (shadow) {
+            E e = cloneEventWithPadding(event, shadow->paddingLeft(), shadow->paddingTop());
+            QCoreApplication::instance()->sendEvent(decoration(), &e);
+        } else {
+            QCoreApplication::instance()->sendEvent(decoration(), event);
+        }
     }
     // Propagate events to parent
     event->ignore();