title: Android studio missing classes (Constraint Layout) (has been resolved)
body: <p>Android studio missing classes.
  The layout is blank.
  <img src="https://i.stack.imgur.com/WemgO.png" alt="BlackJack"></p>
  
  <p>Main.Activity is red letters.</p>
  
  <p><a href="https://i.stack.imgur.com/irs7i.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/irs7i.png" alt="enter image description here"></a></p>
  
  <p>[Soultion in the answer area]</p>