fix: the WKWebView will leak in the platform centered approach unless…

… this property is cleared (apache#1486)

CordovaLib/Classes/Private/Plugins/CDVWebViewEngine/CDVWebViewEngine.m

@@ -273,6 +273,7 @@ - (void)dispose
 {
     WKWebView* wkWebView = (WKWebView*)_engineWebView;
     [wkWebView.configuration.userContentController removeScriptMessageHandlerForName:CDV_BRIDGE_NAME];
+    _engineWebView = nil;
 
     [super dispose];
 }