[gnome-builder/wip/gtk4-port] plugins/html-preview: use add_content_child()

[Christian Hergert <chergert src gnome org>]

commit 538f099d82c74f8784a06f993a8dfdec1c6dcedd
Author: Christian Hergert <chergert redhat com>
Date:   Wed Apr 20 16:54:04 2022 -0700

    plugins/html-preview: use add_content_child()

 src/plugins/html-preview/html_preview.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/plugins/html-preview/html_preview.py b/src/plugins/html-preview/html_preview.py
index 1323d664e..ba9a1cc95 100644
--- a/src/plugins/html-preview/html_preview.py
+++ b/src/plugins/html-preview/html_preview.py
@@ -365,7 +365,7 @@ class HtmlPreviewPage(Ide.Page):
         self.webview.props.hexpand = True
         self.webview.props.vexpand = True
         self.webview.connect('decide-policy', self.on_decide_policy_cb)
-        self.add(self.webview)
+        self.add_content_widget(self.webview)
         self.webview.show()
 
         settings = self.webview.get_settings()