Prevent null reference exception on theme change (#220)

## Test plan
Try changing the VS theme when loading the extension
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Co-authored-by: Tomasz Gołębiowski <tgolebiowski@virtuslab.com>

src/Cody.UI/Controls/WebviewController.cs

@@ -153,6 +153,7 @@ public async void OnThemeChanged(object sender, IColorThemeChangedEvent e)
         {
             try
             {
+                if (_webview == null) return;
                 string updatedScript = e.ThemingScript;
                 if (updatedScript != _colorThemeScript)
                 {