Undefined variable ''vision'' or class ''vision.I​mageDataTy​peConverte​r''

Hello, my realease is Matlab R2018a, Here is the part of error code,

converter = vision.ImageDataTypeConverter;

% opticalflow object

opticalFlow = vision.OpticalFlow('ReferenceFrameDelay', 1);

opticalFlow.OutputValue = 'Horizontal and vertical components in complex form';

Besides, not only the vision.ImageDataTypeConverter but also vision.OpticalFlow has the same issue.

Does anyone can solve this problem?

Thanks!