信号SIGABRT错误：

validateRenderPassDescriptor:611: failed assertion
`No rendertargets set in RenderPassDescriptor.'

下面是我设置MTLRenderPassDescriptor的方法：

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)

调用makeRenderCommandEncoder（）时发生崩溃。
有人知道如何避免崩溃，检查渲染目标是否不存在并中止吗？