Prevent crash if generator exists but has no items

See https://gitlab.com/kicad/code/kicad/-/issues/15366

pcbnew/tools/pcb_selection_tool.cpp

@@ -2367,13 +2367,24 @@ bool PCB_SELECTION_TOOL::itemPassesFilter( BOARD_ITEM* aItem, bool aMultiSelect
         }
     }
 
-    if( aItem->Type() == PCB_GENERATOR_T )
-        aItem = *( static_cast<PCB_GENERATOR*>( aItem )->GetItems().begin() );
-
     if( !aItem )
         return false;
 
-    switch( aItem->Type() )
+    KICAD_T itemType = aItem->Type();
+
+    if( itemType == PCB_GENERATOR_T )
+    {
+        if( static_cast<PCB_GENERATOR*>( aItem )->GetItems().empty() )
+        {
+            wxASSERT_MSG( false, "Tried to select an empty generator" );
+        }
+        else
+        {
+            itemType = ( *static_cast<PCB_GENERATOR*>( aItem )->GetItems().begin() )->Type();
+        }
+    }
+
+    switch( itemType )
     {
     case PCB_FOOTPRINT_T:
         if( !m_filter.footprints )