Had this problem with a blank dialog I had just created. The problem was that I renamed 'dialog.m' to 'dialog.mm' before making a connection. It resolved when I changed the name back to 'dialog.m'. Once a connection was made, I renamed the file again to 'dialog.mm' and could make new connections without problems.

Found it! .clearColor and .loadAction are under drawInMTKView (MTLRenderPassDescriptor*).colorAttachments[0], not under _loadMetalWithView (MTLRenderPipelineDescriptor*).colorAttachments[0]

In 2023, I see neither clearColor nor loadAction under descriptor.colorAttachments[0]. Is there an alternative or will I just have to draw a full-screen polygon?