fix: null m_ixex_mode_combo in TabPrint::clear_pages() to prevent dangling pointer crash

Same class of bug as the TabPrinter fix: clear_pages() destroys all
page widgets but didn't null m_ixex_mode_combo, causing a SIGSEGV in
refresh_ixex_mode_combo() when load_current_preset() ran after a
page rebuild (e.g. triggered by Plater::reset()).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/slic3r/GUI/Tab.cpp

@@ -2903,6 +2903,7 @@ void TabPrint::clear_pages()
 
     m_recommended_thin_wall_thickness_description_line = nullptr;
     m_top_bottom_shell_thickness_explanation = nullptr;
+    m_ixex_mode_combo = nullptr;
 }
 
 //BBS: GUI refactor