title: Swift 3 - 'EXC_BAD_INSTRUCTION(code=EXC_1386_INVOP,subcode=0x0)' Error
body: <p>I am working on a maze game in swift 3 and it compiles but when I run it I get the EXC_BAD_INSTRUCTION error. I'm relatively new to swift so I don't know how to properly debug it. Here is the piece of code that is getting the error:</p>
  
  <pre><code>init(window:SKScene, width:Int, height:Int){
      widthOfBoard = width
      heightOfBoard = height
  
  
      for i in 0..&lt;widthOfBoard {
          var temp:[cell]!
          for j in 0..&lt;heightOfBoard {
              let tempCell = cell(x: i, y: j, boardWidth: widthOfBoard, boardHeight: heightOfBoard, screenSize: window.frame)
              temp.append(tempCell)
          }
          cells.append(temp)
      }
      stackOfCells.append(cells[0][0])
      recursiveMaze(currentX: 0, currentY: 0, window: window)
  }
  </code></pre>
  
  <p>I am getting the error with <code>cells.append(temp)</code>.</p>
  
  <p>I am also getting the error in each of 
  <a href="https://i.stack.imgur.com/9N7OQ.png" rel="nofollow noreferrer">these locations</a>.</p>