Don't update file browser filter if control doesn't exist

When the grid is being validated, SetValue is called, which then ends up
running the file filter update. However, if the cell was never selected,
the file browser button would never have been created.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/16316

common/widgets/grid_text_button_helpers.cpp

@@ -508,8 +508,8 @@ void GRID_CELL_PATH_EDITOR::UpdateFilterString( const wxString& aFilterString )
         m_fileFilter = aFilterString;
 
     // Ensure that the control switches between files and directories properly
-    if( m_fileFilter.IsEmpty() )
-        static_cast<TEXT_BUTTON_FILE_BROWSER*>( m_control )->UpdateFileFilter( nullptr );
-    else
-        static_cast<TEXT_BUTTON_FILE_BROWSER*>( m_control )->UpdateFileFilter( &m_fileFilter );
+    TEXT_BUTTON_FILE_BROWSER* button = dynamic_cast<TEXT_BUTTON_FILE_BROWSER*>( m_control );
+
+    if( button )
+        button->UpdateFileFilter( m_fileFilter.IsEmpty() ? nullptr : &m_fileFilter );
 }