title: How to check if a table element is nil
body: <p>I have a table called <code>frameActions</code> which could not contain some property in some case:</p>
  
  <pre><code>action = 'UP'
  frameActions = {}
  frameActions['UP'] = { a = 1, b = 2 }
  </code></pre>
  
  <p>How do I check if the table have a specific property name?</p>
  
  <pre><code>if frameActions[action].c ~= nil then
      -- do something
  end
  </code></pre>
  
  <p>this condition throws the error: <code>attempt to index a nil value</code></p>