fix: Page and Frame isLoaded undefined checks (#6255)

* fix(view): isLoaded handling

closes https://github.com/NativeScript/NativeScript/issues/6179

* refactor: Error handling code in onCreateView

tns-core-modules/ui/core/view-base/view-base.ts

@@ -595,13 +595,13 @@ export abstract class ViewBase extends Observable implements ViewBaseDefinition
     }
 
     public loadView(view: ViewBase): void {
-        if (!view.isLoaded) {
+        if (view && !view.isLoaded) {
             view.callLoaded();
         }
     }
 
     public unloadView(view: ViewBase): void {
-        if (view.isLoaded) {
+        if (view && view.isLoaded) {
             view.callUnloaded();
         }
     }

tns-core-modules/ui/core/view/view.android.ts

@@ -180,7 +180,7 @@ function initializeDialogFragment() {
             }
 
             const owner = this.owner;
-            if (!owner.isLoaded) {
+            if (owner && !owner.isLoaded) {
                 owner.callLoaded();
             }
 
@@ -196,7 +196,7 @@ function initializeDialogFragment() {
             }
 
             const owner = this.owner;
-            if (owner.isLoaded) {
+            if (owner && owner.isLoaded) {
                 owner.callUnloaded();
             }
         }

tns-core-modules/ui/frame/frame.android.ts

@@ -9,7 +9,7 @@ import { Page } from "../page";
 import * as application from "../../application";
 import {
     FrameBase, stack, goBack, View, Observable,
-    traceEnabled, traceWrite, traceCategories
+    traceEnabled, traceWrite, traceCategories, traceError
 } from "./frame-common";
 
 import {
@@ -696,8 +696,23 @@ class FragmentCallbacksImplementation implements AndroidFragmentCallbacks {
         }
 
         const entry = this.entry;
+        if (!entry) {
+            traceError(`${fragment}.onCreateView: entry is null or undefined`);
+            return null;
+        }
+
         const page = entry.resolvedPage;
+        if (!page) {
+            traceError(`${fragment}.onCreateView: entry has no resolvedPage`);
+            return null;
+        }
+
         const frame = this.frame;
+        if (!frame) {
+            traceError(`${fragment}.onCreateView: this.frame is null or undefined`);
+            return null;
+        }
+
         if (page.parent === frame) {
             // If we are navigating to a page that was destroyed
             // reinitialize its UI.
@@ -706,12 +721,12 @@ class FragmentCallbacksImplementation implements AndroidFragmentCallbacks {
                 page._setupUI(context);
             }
         } else {
-            if (!this.frame._styleScope) {
+            if (!frame._styleScope) {
                 // Make sure page will have styleScope even if parents don't.
                 page._updateStyleScope();
             }
 
-            this.frame._addView(page);
+            frame._addView(page);
         }
 
         if (frame.isLoaded && !page.isLoaded) {

tns-core-modules/ui/styling/style-scope.ts

@@ -357,7 +357,7 @@ export class CssState {
      * As a result, at some point in time, the selectors matched have to be requerried from the style scope and applied to the view.
      */
     public onChange(): void {
-        if (this.view.isLoaded) {
+        if (this.view && this.view.isLoaded) {
             this.unsubscribeFromDynamicUpdates();
             this.updateMatch();
             this.subscribeForDynamicUpdates();