Error building Gantt Chart in LWC

I was able to get a working test chart using ChartJS but I’m having trouble with the Frappe Gantt library.

Right now it’s throwing an error in the catch, but I think the error is null as I’m getting

Cannot read property ‘message’ of undefined

If I force it to call Initializechartjs(), I then get

Gantt is not defined

I can see all 4 resources loading –

Here is my code

chart.html

<template>
    <lightning-card title="Gantt">
        <svg class="gantt" width="400" height="450" lwc:dom="manual"></svg>
    </lightning-card>
</template>

chart.js

import { LightningElement,api } from 'lwc';
import { ShowToastEvent } from 'lightning/platformShowToastEvent';
import { loadScript, loadStyle } from 'lightning/platformResourceLoader';
import momentJS from '@salesforce/resourceUrl/momentJS';
import snapSVG from '@salesforce/resourceUrl/snapSVG';
import frappeGanttMin from '@salesforce/resourceUrl/frappeGanttMin';
import frappeGanttStyle from '@salesforce/resourceUrl/frappeGanttStyle';


 export default class Chartcmp extends LightningElement {
  @api chartjsInitialized = false;
   @api recordId;
   renderedCallback() {
        if (this.chartjsInitialized) {
          return;
         }
        this.chartjsInitialized = true;
        console.log('chart loading');
        Promise.all([
              loadScript(this, momentJS),
              loadScript(this, snapSVG),
              loadScript(this, frappeGanttMin),
              loadStyle(this, frappeGanttStyle)
        ])
    .then(() => {
        this.Initializechartjs();
    })
    .catch(error => {
        this.dispatchEvent(
            new ShowToastEvent({
                title: 'Error loading chart',
                message: error.message,
                variant: 'error'
            })
        );
    });
   }

  Initializechartjs() {
    console.log("loaded");
    //Get the context of the canvas element we want to select
    var chartClass = this.template.querySelector(".gantt");
    var gantt = new Gantt(chartClass ,{
        tasks: [
            {
                id: 'Task 1',
                name: 'Redesign website',
                start: '2016-12-28',
                end: '2016-12-31',
            },
            {
                id: 'Task 2',
                name: 'Second Task',
                start: '2016-12-29',
                end: '2016-12-30',
            }
        ]
    });

    }

}