Error HTTP request on Android 9 Nougat:
http failure response for url 0 unknown error
Fix:
In AndroidManifest.xml set usesCleartextTraffic = true on application element:
<application
android:name="com.tns.NativeScriptApplication"
android:allowBackup="true"
android:usesCleartextTraffic="true"
android:icon="@drawable/icon"
android:label="@string/app_name"
android:theme="@style/AppTheme">