Cannot read properties of null (reading 'getContext') at new EchartsDropdownComponent

You should place the chart creation code inside the method ngAfterViewInit and don’t forget to specify the chart type.

export class EchartsDropdownComponent implements AfterViewInit, OnChanges {

  ngAfterViewInit() {
    myChart = new Chart('myChart', {
      type: 'scatter', // <- define the chart type
      data: {
        datasets: [{
          label: 'Scatter Dataset',
          ...
  }