do not use unique_ptr for QGraphicsItem

since the ownership of the item is transferred to the scene
this leads to a crash when deleting the object holding the pointer

since the ownership is transferred back when removing the item, one should
take care of resetting the pointer no canvas deletion

this should fix #28962

src/gui/qgssnapindicator.cpp

@@ -27,24 +27,43 @@
 QgsSnapIndicator::QgsSnapIndicator( QgsMapCanvas *canvas )
   : mCanvas( canvas )
 {
+  mCanvasDestroyedConnection = QObject::connect( canvas, &QgsMapCanvas::destroyed, [ = ]()
+  {
+    mCanvas = nullptr;
+    mSnappingMarker = nullptr;
+  } );
 }
 
-QgsSnapIndicator::~QgsSnapIndicator() = default;
+QgsSnapIndicator::~QgsSnapIndicator()
+{
+  if ( mSnappingMarker && mCanvas )
+  {
+    mCanvas->scene()->removeItem( mSnappingMarker );
+    delete mSnappingMarker;
+  }
+
+  QObject::disconnect( mCanvasDestroyedConnection );
+};
 
 void QgsSnapIndicator::setMatch( const QgsPointLocator::Match &match )
 {
   mMatch = match;
 
   if ( !mMatch.isValid() )
   {
-    mSnappingMarker.reset();
+    if ( mSnappingMarker )
+    {
+      mCanvas->scene()->removeItem( mSnappingMarker );
+      delete mSnappingMarker; // need to delete since QGraphicsSene::removeItem transfers back ownership
+    }
+    mSnappingMarker = nullptr;
     QToolTip::hideText();
   }
   else
   {
     if ( !mSnappingMarker )
     {
-      mSnappingMarker.reset( new QgsVertexMarker( mCanvas ) );
+      mSnappingMarker = new QgsVertexMarker( mCanvas ); // ownership of the marker is transferred to QGraphicsScene
       mSnappingMarker->setIconSize( QgsGuiUtils::scaleIconSize( 10 ) );
       mSnappingMarker->setPenWidth( QgsGuiUtils::scaleIconSize( 3 ) );
     }

src/gui/qgssnapindicator.h

@@ -56,7 +56,8 @@ class GUI_EXPORT QgsSnapIndicator
 
     QgsMapCanvas *mCanvas;
     QgsPointLocator::Match mMatch;
-    std::unique_ptr<QgsVertexMarker> mSnappingMarker;
+    QgsVertexMarker *mSnappingMarker = nullptr;
+    QMetaObject::Connection mCanvasDestroyedConnection;
 };
 
 #endif // QGSSNAPINDICATOR_H