Cannot set user parameters in BackgroundSubtractorMOG2

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OpenCV library version 2.42. I'd like to set a parameter in BackgroundSubtractorMOG2 object, e.g.

BackgroundSubtractorMOG2 bgr;

// the following doesn't work because 'nmixtures', 'backgroundRatio' 
// and 'fVarMin' are a protected members.
bgr.nmixtures = 3;   
bgr.backgroundRatio = 0.9;
bgr.fVarMin = 5;

// the following works 
bgr.set('nmixtures', 3); 

// However, the following lines will give a run-time error 
// `Access violation reading location 0x0000000000000008.`
bgr.set("backgroundRatio", 0.9);  
bgr.set("backgroundRatio", (float) 0.9);
bgr.set("fVarMin", 5);

backgroundRatio and fVarMin are parameters that control the algorithm. A user should be able to change these parameters after constructing the instance of BackgroundSubtractorMOG2 class as stated in the documentation.

How can I set the parameters of BackgroundSubtractorMOG2?