I'm trying to embed a Visualforce Page onto my home page that allows users to select a report and filter it right there. 

When running I seem to be getting the following error:
caused by: System.NullPointerException: Attempt to de-reference a null object

Class.AnalyticsController.getSelectedFilters: line 69, column 1
Class.AnalyticsController.getChartFilter: line 64, column 1

Can anyone identify what's causing this issue? I notice this code was written for API Ver 29, while I'm on 48. 

Visualforce Page: 

<apex:page controller="AnalyticsController">
<style>
label {
    font-weight: bold;
}
#filters {
    overflow: hidden;
    width: 100%
}
#filterBox {
    float: left;
    align: center;
    padding: 5px 5px 5px 0px;
}
</style>
<apex:form >
    <apex:outputLabel value="Select Report"/> 
    <apex:selectList value="{!reportId}" multiselect="false" size="1">
        <apex:selectOptions value="{!availableReports}"/>
    </apex:selectList>
    <apex:commandButton action="{!getReportInfo}" value="Get Report Filters" reRender="report"/><br/>

    <apex:outputPanel id="report" layout="block">
        <apex:outputPanel rendered="{!reportId != null}">
            <div id="filters">
                <apex:repeat value="{!availableColumnFilters}" var="colFilter">
                    <div id="filterBox">
                        <apex:outputLabel >{!colFilter.label}</apex:outputLabel><br/>
                        <apex:selectList value="{!colFilter.operator}" size="1" multiselect="false" style="width: 100px;">
                            <apex:selectOption itemLabel="--None--" itemValue=""/>
                            <apex:selectOptions value="{!availableDataTypeFilterOperators[colFilter.dataType]}"/>
                        </apex:selectList>
                        <apex:inputText value="{!colFilter.value}"/>
                    </div>   
                </apex:repeat>
            </div>

            <apex:commandButton value="Get Chart with Filters" reRender="chart"/><br/>

            <apex:outputPanel layout="block" id="chart">
                <analytics:reportChart reportId="{!reportId}" filter="{!chartFilter}"/>
            </apex:outputPanel>
        </apex:outputPanel>
    </apex:outputPanel>
</apex:form>
</apex:page>

public with sharing class AnalyticsController{
 
    public List<SelectOption> availableReports { get; set; }
    public Id reportId { get; set; }
    public Map<String, List<SelectOption>> availableDataTypeFilterOperators { get; set; }
    public List<ColumnFilter> availableColumnFilters { get; set; }
 
    public AnalyticsController() {
        availableReports = retrieveAvailableReports();
        availableDataTypeFilterOperators = retrieveAvailableDataTypeFilterOperators();
    }
 
    public List<SelectOption> retrieveAvailableReports() {
        List<SelectOption> reptOpts = new List<SelectOption>();
        for (Report r : [
             Select Id, Name
             From Report
             Where Format In ('Summary','Matrix')
             Order By Name
        ]) {
            reptOpts.add(new SelectOption(r.Id, r.Name));
        }
        return reptOpts;
    }
 
    public Map<String, List<SelectOption>> retrieveAvailableDataTypeFilterOperators() {
        Map<String, List<SelectOption>> dataTypeFilterOpts = new Map<String, List<SelectOption>>();
        Map<String, List<Reports.FilterOperator>> filterOperatorMap = Reports.ReportManager.getDataTypeFilterOperatorMap();
 
        for (String dataType : filterOperatorMap.keySet()) {
            List<SelectOption> operators = new List<SelectOption>();
 
            // Append _DATA to match ColumnDataType from ReportTypeColumn
            dataTypeFilterOpts.put(dataType.toUpperCase() + '_DATA', operators);
 
            for (Reports.FilterOperator fo : filterOperatorMap.get(dataType)) {
                operators.add(new SelectOption(fo.getName(), fo.getLabel()));
            }
        }        
        return dataTypeFilterOpts;
    }
 
    public PageReference getReportInfo() {
        Reports.ReportDescribeResult descRes = Reports.ReportManager.describeReport(reportId);
 
        availableColumnFilters = new List<ColumnFilter>();
 
        for (Reports.ReportTypeColumnCategory category : descRes.getReportTypeMetadata().getCategories()) {
            for (Reports.ReportTypeColumn col : category.getColumns().values()) {
                if (col.getFilterable()) {
                    ColumnFilter cf = new ColumnFilter(
                        col.getLabel(),
                        col.getName(),
                        col.getDataType().name()
                    );
                    availableColumnFilters.add(cf);
                }
            }
        }
        return null;
    }
 
    public String getChartFilter() {
        return JSON.serialize(getSelectedFilters());
    }
 
    private List<ColumnFilter> getSelectedFilters() {
        List<ColumnFilter> selectedFilters = new List<ColumnFilter>();
        for (ColumnFilter cf : availableColumnFilters) {
            if (String.isNotBlank(cf.operator)) {
                selectedFilters.add(cf);
            }
        }
        return selectedFilters;
    }
 
    public class ColumnFilter {
        public ColumnFilter(String lab, String col, String dt) {
            label = lab;
            column = col;
            dataType = dt;
        }
 
        // Values needed for apex:analytics component
        public String column { get; set; }
        public String operator { get; set; }
        public String value { get; set; }
 
        // Values need for display and operator select list
        public String label { get; set; }
        public String dataType { get; set; }
 
    }
}