Can't read properties of null in javaScript in MS CRM

function ShowHideTabs(executionContext) {

//Get the form contex
var formContext = executionContext.getFormContext();
var optionSetAttribute = formContext.getAttribute("mri_optionset").getValue();

if (formContext.getAttribute("mri_optionset").getValue() == 171800000) {
formContext.ui.tabs.get("tab_2").setVisible(true);

}
else (formContext.getAttribute("mri_optionset").getValue() != 0){
formContext.ui.tabs.get("tab_2").setVisible(false);
}
}