title: No rendertargets set in RenderPassDescriptor
body: <p>The signal SIGABRT error:</p>
  
  <pre><code>validateRenderPassDescriptor:611: failed assertion
  `No rendertargets set in RenderPassDescriptor.'
  </code></pre>
  
  <p>This is how I set up the MTLRenderPassDescriptor:</p>
  
  <pre><code>guard let currenDrawable: CAMetalDrawable = self.currentDrawable else { return }
  
  let renderPassDescriptor = MTLRenderPassDescriptor()
  renderPassDescriptor.colorAttachments[0].texture = currenDrawable.texture
  renderPassDescriptor.colorAttachments[0].loadAction = .clear
  renderPassDescriptor.colorAttachments[0].clearColor = MTLClearColor(red: 0, green: 0, blue: 0, alpha: 0)            
  
  let renderCommandEncoder = commandBuffer.makeRenderCommandEncoder(descriptor: renderPassDescriptor)
  </code></pre>
  
  <p>The crash occurs when I call makeRenderCommandEncoder().</p>
  
  <p>Anyone know how to avoid a crash, to check if rendertargets are not present and abort?</p>