Find error when error message doesn't show location

Question

I'm building an app using Ionic, Angular, and Firebase.

I'm having a problem where the code is being built and packaged by webpack and it returns an error from polyfills.js, which is not useful in the slightest...

The error message looks like this:

Unhandled Promise rejection: Missing Command Error ; Zone: <root> ; Task:
Promise.then ; Value: Missing Command Error undefined

The error happens every 30ish seconds.

And the non-minifed version of the error looks like this:

function l(t) {
    var e = t.prototype,
        n = e.then;
    e[d] = n;
    var r = Object.getOwnPropertyDescriptor(t.prototype, "then");
    r && !1 === r.writable && r.configurable && Object.defineProperty(t.prototype, "then", {
            writable: !0
        }),
        t.prototype.then = function(t, e) {
            var r = this;
            return new x(function(t, e) {
                n.call(r, t, e)
            }).then(t, e)
        },
        t[R] = !0
}
var h = n.symbol,
    p = [],
    v = h("Promise"),
    d = h("then");
n.onUnhandledError = function(t) {
        if (n.showUncaughtError()) {
            var e = t && t.rejection;
            e ? console.error("Unhandled Promise rejection:", e instanceof Error ? e.message : e, "; Zone:", t.zone.name, "; Task:", t.task && t.task.source, "; Value:", e, e instanceof Error ? e.stack : void 0) : console.error(t)
        }
    },
    n.microtaskDrainDone = function() {
        for (; p.length;)
            for (; p.length;)
                ! function() {
                    var t = p.shift();
                    try {
                        t.zone.runGuarded(function() {
                            throw t
                        })
                    } catch (t) {
                        r(t)
                    }
                }()
    };

Any idea how I would debug this? Thank you!