Don't call unimplemented virtual wxListCtrl::OnGetItemIsChecked()

This method should only be called when checkboxes are enabled and
calling it unconditionally triggered an assert failure.

See #18393.

Closes https://github.com/wxWidgets/wxWidgets/pull/1330

src/generic/listctrl.cpp

@@ -1681,7 +1681,11 @@ void wxListMainWindow::CacheLineData(size_t line)
         ld->SetImage(col, listctrl->OnGetItemColumnImage(line, col));
     }
 
-    ld->Check(listctrl->OnGetItemIsChecked(line));
+    if ( HasCheckBoxes() )
+    {
+        ld->Check(listctrl->OnGetItemIsChecked(line));
+    }
+
     ld->SetAttr(listctrl->OnGetItemAttr(line));
 }