Don't crash if wxWebView::EnableContextMenu() is called too early

Although caller of wxWebViewEdgeImpl::GetSettings() check if it returns
NULL and handle it gracefully, this function itself can crash if it's
called too early, i.e. if ICoreWebView2 hasn't been created yet.

Add a check to avoid this and just return NULL settings in this case.

Closes #18840.

src/msw/webview_edge.cpp

@@ -325,6 +325,9 @@ HRESULT wxWebViewEdgeImpl::OnWebViewCreated(HRESULT result, ICoreWebView2Control
 
 ICoreWebView2Settings* wxWebViewEdgeImpl::GetSettings()
 {
+    if (!m_webView)
+        return NULL;
+
     ICoreWebView2Settings* settings;
     HRESULT hr = m_webView->get_Settings(&settings);
     if (FAILED(hr))