I had a quick look at your error, looks like it was often reported (see below)
I quickly went thru them, I kind of have the feeling that often the problem was
a. Too old plugins or cordova-android version
or
b. Having cordova-android@7.0.0 and using old plugins
or
c. Not having a local grade installed (actually I didn’t read that one but it’s also a classic one)
or
d. sometimes removing and adding the platform after having played with the plugins help (that one is also from me because I just had to do it 
)
Do one of these apply to your app?
I know probably not the answer you wanted to hear but maybe it could lead you to something, at least I hope it helps a bit.
Here the list of issues:
https://forum.ionicframework.com/search?q=UnhandledPromiseRejectionWarning%20order%3Alatest