Yes, the warning is justified: You're calling alert within a microtask, in this case a promise completion. (See Difference between microtask and macrotask within an event loop context.)

alert, prompt, and confirm are relics of an age long past, and they have a problem: They completely interrupt the normal workings of JavaScript and arguably violate its run-to-completion semantics by completely suspending execution, right in the middle of a job from the job queue (or a task from the event loop; JavaScript and the HTML5 spec differ on terminology) and do UI in the form of a blocking modal. This is just not in keeping with the general mode of interaction, which is event-based (show the message, get an event when it's closed).

You can work around it by doing the alert in a macrotask instead:

this.service.uploadFile((error: any) => {
    if (error) {
        setTimeout(() => {
            console.log(error);
            alert("An error occured");
        }, 0);
        return;
    }

    this.onUploadCompleted()
});

...but really the solution is to stop using alert, prompt, and confirm entirely.

Here's a fun example of micro and macro tasks: A promise completion is a microtask, whereas timer callbacks are macrotasks. The initial run of the script is also a macrotask. All of the microtasks queued by a macrotask are run before the next macrotask is run; e.g., they jump the queue. So with this:

// First, we set a timer callback for 0ms
setTimeout(() => {
  console.log("timer fired");
}, 0);

// Now we get a promise that's *already* resolved and hook a callback
Promise.resolve().then(() => {
  console.log("promise resolved");
});

// Now show a message demonstrating we got here before the promise resolution callback
console.log("got to the end");

...we see

got to the end
promise resolved
timer fired

...rather than

got to the end
timer fired
promise resolved

....which we would have gotten if all tasks were created equal.