an exception occurs when calling getFeatureType

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I want to get the CascadeClassifier type by calling the member function getFeatureType. But the code throws an access exception. The exception clue are Chinese characters, and the exception describes that 0x60B9CA49 (opencv_objdetect2410d.dll) (in TestFaceDetection.exe) has an unhandled exception: 0xC0000005: An access violation occurred while reading location 0x00000000. How to fix this problem, the code fragment lists following

CascadeClassifier cas;
cas.load("xxx.xml");//xxx.xml is a cascadeclassifier xml file
int feature_type = cas.getFeatureType();