Fix crash on styler dialog of User Defined Language dialog

Fixes #2646, fixes #4215, close #4279

PowerEditor/src/ScitillaComponent/UserDefineDialog.cpp

@@ -1667,6 +1667,9 @@ INT_PTR CALLBACK StylerDlg::dlgProc(HWND hwnd, UINT message, WPARAM wParam, LPAR
 
         case WM_COMMAND :
         {
+			if (dlg == nullptr)
+				return FALSE;
+
             Style & style = SharedParametersDialog::_pUserLang->_styleArray.getStyler(dlg->_stylerIndex);
             if (HIWORD(wParam) == CBN_SELCHANGE)
             {