Report Beforefactory chart drop errors in interactivity

I am working on the report which changes the design mased on user input date range params which is set in initialize. I wrote a script in beforefactory which drop 2 charts based on the daterange param and only render and execute only 1 chart and corresponding dataset.

Report execute successfully but during interactivity, any action would throw exception of cannot execute beforefactory script below:

DaysChart = reportContext.getReportRunnable().designHandle.getDesignHandle().findElement("dayschart");
HoursChart = reportContext.getReportRunnable().designHandle.getDesignHandle().findElement("hourschart");
WeeksChart = reportContext.getReportRunnable().designHandle.getDesignHandle().findElement("weekschart");


if (chartType == "days"){
HoursChart.drop();WeeksChart.drop();
}
else if( chartType == "hours"){
WeeksChart.drop();DaysChart.drop();
}
else if(chartType == "weeks"){
HoursChart.drop();DaysChart.drop();
}
else{
}

How do i change this script so that this run only with the initial report generation/rendering and not during with any interactivity action.

Thanks
Abhishek