So I actually just ran into the exact same problem today except without a modal involved. In my form, I have two buttons. One that submits the form and one that, when clicked, routes back to the previous page.
<button class="btn btn-default" routerLink="/events">Cancel</button>
<button type="submit" class="btn btn-primary">Submit</button>
Clicking on the first button with the routerLink does exactly what its supposed to, but also apparently tries to submit the form as well, and then has to abandon form submission because the page that the form was on is unmounted from the DOM during submission.
This appears to be the exact same thing that is happening to you, except with a modal instead of the entire page.
The problem becomes fixed if you directly specify the type of the second button to be something other than submit.
<button type="button "class="btn btn-default" routerLink="/events">Cancel</button>
So if you are closing the modal via a 'Cancel' button or something of the sort, specifying that button's type, as shown above, should solve your issue.