When using 'Ext.grid.feature.Grouping' with 'enableGroupingMenu: false' config parameter there is an error in ext-all-debug.js at line 79141 when toggling grouping feature enable/disable. The error 'groupToggleMenuItem is null' is at line:

Code:
groupToggleMenuItem.setChecked(true, true);
So, you just need to add a check for 'groupToggleMenuItem is not null' to resolve this.