I don't understand: If you're able to detect an empty selectedKey() for the current combobox, then i don't understand why you couldn't do it for the other combobox, and thus solving your problem.......?
In the second combobox, modify the 'change' event handler to:
change:function(){
if (mirCategoryCombo.getSelectedKey() == null || mirCategoryCombo.getSelectedKey == "") {
// your alert here
}
else {
if(this.getSelectedKey()=="" || this.getSelectedKey()==null){
this.setValueState(sap.ui.core.ValueState.Error);
raiseNotification(5,20019,"","","","","I");
}else{
this.setValueState(sap.ui.core.ValueState.None);
}
mirController.mirEmptyProductControls("mirProductGroupCombo");
}
but I think a much better UX would be to simply disable the 2nd combobox if nothing is selected in the 1st combobox, and enable it once you have selected something in the 1st combobox.