Drats.. it seems that we created almost the same bug in 20.0 as we had in 19.0. The fix below should work for 20.0.

Code:
// BUGFIX: WebSet of psMask before control is rendered causes JavaScript error, this will be fixed post 20.0.
if(df.pnDataFlexVersion == 20.0){
df.WebForm.prototype._initMask = df.WebForm.prototype.initMask;
df.WebForm.prototype.initMask = function(){
    if(this._eControl){
        this._initMask();
    }
}
}