Sidenav is initialized right after the js code loads. I suppose in your case the js code is loaded first, and then Sidenav is added.

You can check it easily. Just check what returns mdb.Sidenav.getInstance (document.getElementById ('sidenav'));. If it returns undefined, Sidenav has not been initialized.

The safest thing to do is to check if there is an instance of sidenav and add it if necessary. Try adding such code in the AfterViewInit hook:

let instance = mdb.Sidenav.getInstance(this._elementRef.nativeElement;);

if (!instance) {
    instance = new mdb.Sidenav(this._elementRef.nativeElement;);
}