title: Is this ok to run act.runOnUIThread when it's not a currently visible activity?
body: <p>Let's say that I have a <code>mainActivity</code> and a reference to it everywhere.</p>
  
  <p>I switch activities later in the app and I'm not sure which is currently 'on the top' and if it's a <code>mainActivity</code>.</p>
  
  <p>Is it wrong to call <code>mainActivity.runOnUIThread()</code> when there's another activity visible? Would it work without any errors in the future? Or should I call it only on the current activity?</p>
  
  <p>Technically, there's one UI thread so...</p>