Display dropdown values based on other dropdown value

i’m trying to display drop-down area based on other dropdown city id using API’s … but i fail and getting this error
SpecViewPage.html:32 ERROR Error: Cannot find a differ supporting object 'empty' of type 'string'. NgFor only supports binding to Iterables such as Arrays. at NgForOf.ngOnChanges (VM58 vendor.js:48829) at checkAndUpdateDirectiveInline (VM58 vendor.js:12720) at checkAndUpdateNodeInline (VM58 vendor.js:14248) at checkAndUpdateNode (VM58 vendor.js:14191) at debugCheckAndUpdateNode (VM58 vendor.js:15084) at debugCheckDirectivesFn (VM58 vendor.js:15025) at Object.eval [as updateDirectives] (VM156 SpecViewPage.ngfactory.js:414) at Object.debugUpdateDirectives [as updateDirectives] (VM58 vendor.js:15010) at checkAndUpdateView (VM58 vendor.js:14157) at callViewAction (VM58 vendor.js:14508)
my code