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findHomography might return an empty Mat, if it could not find a good transformation, so you need to check for that !

then it's just maths:

        if np.shape(M) == ():
            print( "No transformation possible" )
            return None, None

        ## derive rotation angle from homography
        theta = - math.atan2(M[0,1], M[0,0]) * 180 / math.pi