Dear @Ken Wilson

Make sure you use correctly @ViewChild decorator to get access to MdbTabset methods.

Here is my code with example, and everything works correctly (I don't get any error)

HTML:

<div class="container">
  <mdb-tabset #mainTabs [buttonClass]="'md-tabs tabs-orange tabs-3'" [contentClass]="'card'">
    <mdb-tab heading="Main tab">
      <div>Main Tab Content</div>
      <mdb-tabset #subTabs [buttonClass]="'md-tabs tabs-orange tabs-3'" [contentClass]="'card'">
        <mdb-tab heading="Sub tab">
          <div>Sub tab content</div>
          <mdb-tabset
            #selectedModelSubTabs
            [buttonClass]="'md-tabs tabs-orange tabs-3'"
            [contentClass]="'card'"
          >
            <mdb-tab *ngFor="let tab of tabs" heading="{{ tab.title }}">
              {{ tab.content }}
            </mdb-tab>
          </mdb-tabset>
        </mdb-tab>
      </mdb-tabset>
    </mdb-tab>

    <mdb-tab *ngFor="let tab of tabs" heading="{{ tab.title }}">
      {{ tab.content }}
    </mdb-tab>
  </mdb-tabset>
</div>

TS:

import { TabsetComponent } from 'your_path';
  @ViewChild('mainTabs', { static: true }) mainTabs: TabsetComponent;
  @ViewChild('subTabs', { static: true }) subTabs: TabsetComponent;
  @ViewChild('selectedModelSubTabs', { static: true }) selectedModelSubTabs: TabsetComponent;
  tabs: any[] = [{ title: 'Tab1', content: 'content 1' }, { title: 'Tab2', content: 'content 2' }];

  ngAfterViewInit() {
    this.mainTabs.setActiveTab(1);

    this.subTabs.setActiveTab(1);

    this.selectedModelSubTabs.setActiveTab(2);
  }

Hope it helps! Best Regards, Bartosz.

please see attached snippet lines 7 (#customTabs) and then on 111 (#modelTabs) and third set line 298 (#selectedModelSubTabs).