error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

Record some mistakes when using cv2 commit, so as not to forget later.
Here is the error operation code appears, the code I am running is to use semantic segmentation on the road dataset classification process, need to be detected classification entered the picture, and then there was such a mistake, because there is no input images you find, sometimes using the camera time will often report this error, this time need to check the picture address you entered is correct, or if the camera can normally open.