I was also getting the same error for last two days and finally i caught the culprit.
This error started showing after upgrading the AngularFire2 to 5.0.0-rc.8 or higher and the reason is
rxjs 5 is no longer supported in AngularFire2 5.0.0-rc.8 or higher, upgrade to 6 and include rxjs-compat
npm i rxjs@^6.0 rxjs-compat