fix(modal): lower isModalNavigation flag when closing modal (#1378)

Author: ADjenkov

Diff for: nativescript-angular/router/ns-location-strategy.ts

@@ -328,6 +328,8 @@ export class NSLocationStrategy extends LocationStrategy {
         if (!this._isModalClosing) {
             throw new Error("Calling startCloseModal while not closing modal.");
         }
+
+        this._isModalNavigation = false;
         this._isModalClosing = false;
     }