Google Docs Script to count number of cells in a sheet that match with a specified background colour

countMatchingColoredCells.gs

/**
* @param {range} countRange Range to be evaluated
* @param {range} colorRef Cell with background color to be searched for in countRange
* @return {number}
* @customfunction
*/

function countMatchingColoredCells(countRange,colorRef) {
  var activeRange = SpreadsheetApp.getActiveRange();
  var activeSheet = activeRange.getSheet();
  var formula = activeRange.getFormula();
  
  var rangeA1Notation = formula.match(/\((.*)\,/).pop();
  var range = activeSheet.getRange(rangeA1Notation);
  var bg = range.getBackgrounds();
  var values = range.getValues();
  
  var colorCellA1Notation = formula.match(/\,(.*)\)/).pop();
  var colorCell = activeSheet.getRange(colorCellA1Notation);
  var color = colorCell.getBackground();
  
  var count = 0;
  
  for(var i=0;i<bg.length;i++)
    for(var j=0;j<bg[0].length;j++)
      if( bg[i][j] == color )
        count=count+1;
  return count;
};

Hello...

I'm receiving the following error:
Error
TypeError: Cannot read property 'pop' of null (line 13).

Help ?