How do I hide a menu item in the actionbar?

Get a MenuItem pointing to such item, call setVisible on it to adjust its visibility and then call invalidateOptionsMenu() on your activity so the ActionBar menu is adjusted accordingly.

Update: A MenuItem is not a regular view that's part of your layout. Its something special, completely different. Your code returns null for item and that's causing the crash. What you need instead is to do:

MenuItem item = menu.findItem(R.id.addAction);

Here is the sequence in which you should call: first call invalidateOptionsMenu() and then inside onCreateOptionsMenu(Menu) obtain a reference to the MenuItem (by calling menu.findItem()) and call setVisible() on it

Found an addendum to this question:

If you want to change the visibility of your menu items on the go you just need to set a member variable in your activity to remember that you want to hide the menu and call invalidateOptionsMenu() and hide the items in your overridden onCreateOptionsMenu(...) method.

//anywhere in your code
...
mState = HIDE_MENU; // setting state
invalidateOptionsMenu(); // now onCreateOptionsMenu(...) is called again
...

@Override
public boolean onCreateOptionsMenu(Menu menu)
{
    // inflate menu from xml
    MenuInflater inflater = getSupportMenuInflater();
    inflater.inflate(R.menu.settings, menu);

    if (mState == HIDE_MENU)
    {
        for (int i = 0; i < menu.size(); i++)
            menu.getItem(i).setVisible(false);
    }
}

In my example I've hidden all items.