You missed the * in front of NgIf (like we all have, dozens of times):

<div *ngIf="answer.accepted">&#10004;</div>

Without the *, Angular sees that the ngIf directive is being applied to the div element, but since there is no * or <template> tag, it is unable to locate a template, hence the error.

If you get this error with Angular v5:

Error: StaticInjectorError[TemplateRef]:
  StaticInjectorError[TemplateRef]:
    NullInjectorError: No provider for TemplateRef!

You may have <template>...</template> in one or more of your component templates. Change/update the tag to <ng-template>...</ng-template>.

So, I can help to understand what (and where) you made the mistake, but before it, we need to read the documentation:

Access a TemplateRef instance by placing a directive on an element (or directive prefixed with *). The TemplateRef for the embedded view is injected into the constructor of the directive, using the TemplateRef token.

You can also use a Query to find a TemplateRef associated with a component or a directive.

So:

• When using [ngIf]="something" you must use <ng-template> to access the template ref (eg: when using <template> or <ng-template>);
• Using *ngIf you can use it where you want 'cause the * garante the access to template ref(eg: when using <span> or <div>);

If you're using in your code something like <span [ngIf]="message"> {{message}}</span> you probably gonna get some error.