Select the MASH network and run this simple script:

//MASH Placer Clearer
{
    string $sel[] = `ls -sl`;
    
    if (($sel[0] + "_Placer.paintJson") !=0)
    {
        setAttr ($sel[0] + "_Placer.paintJson") -type "string" "{}";
    }
    else
    {
        print "There is no Placer node in this MASH!";
    }
}

Cheers, D