[webview_flutter_wkwebview] Prevents NSObject.removeObserver from being called without calling addObserver first (#5975)

Author: bparrishMines

packages/webview_flutter/webview_flutter_wkwebview/lib/src/web_kit_webview_widget.dart

@@ -93,6 +93,7 @@ class WebKitWebViewPlatformController extends WebViewPlatformController {
 
   bool _zoomEnabled = true;
   bool _hasNavigationDelegate = false;
+  bool _progressObserverSet = false;
 
   final Map<String, WKScriptMessageHandler> _scriptMessageHandlers =
       <String, WKScriptMessageHandler>{};
@@ -455,17 +456,20 @@ class WebKitWebViewPlatformController extends WebViewPlatformController {
     await _resetUserScripts(removedJavaScriptChannels: javascriptChannelNames);
   }
 
-  Future<void> _setHasProgressTracking(bool hasProgressTracking) {
+  Future<void> _setHasProgressTracking(bool hasProgressTracking) async {
     if (hasProgressTracking) {
-      return webView.addObserver(
+      _progressObserverSet = true;
+      await webView.addObserver(
         webView,
         keyPath: 'estimatedProgress',
         options: <NSKeyValueObservingOptions>{
           NSKeyValueObservingOptions.newValue,
         },
       );
-    } else {
-      return webView.removeObserver(webView, keyPath: 'estimatedProgress');
+    } else if (_progressObserverSet) {
+      // Calls to removeObserver before addObserver causes a crash.
+      _progressObserverSet = false;
+      await webView.removeObserver(webView, keyPath: 'estimatedProgress');
     }
   }
 

packages/webview_flutter/webview_flutter_wkwebview/test/src/web_kit_webview_widget_test.dart

@@ -1195,6 +1195,16 @@ void main() {
 
         verify(mockCallbacksHandler.onProgress(32));
       });
+
+      testWidgets('progress observer is not removed without being set first',
+          (WidgetTester tester) async {
+        await buildWidget(tester, hasProgressTracking: false);
+
+        verifyNever(mockWebView.removeObserver(
+          mockWebView,
+          keyPath: 'estimatedProgress',
+        ));
+      });
     });
 
     group('JavascriptChannelRegistry', () {