fix: check null value

Author: shijistar

.storybook/preview.tsx

@@ -17,7 +17,7 @@ const preview: Preview = {
   decorators: [
     (Story, context) => {
       const bgValue = context.globals?.backgrounds?.value;
-      const isDark = getColorLuminance(bgValue) < 0.5;
+      const isDark = bgValue && getColorLuminance(bgValue) < 0.5;
       return (
         <AntdConfigProvider theme={{ algorithm: isDark ? theme.darkAlgorithm : theme.defaultAlgorithm }}>
           <AntdApp>