fix legacy app client download links

jam-ui/src/components/public/JKDownloadsLegacy.js

@@ -72,8 +72,9 @@ const JKDownloadsLegacy = () => {
 
     const downloadLink = React.useMemo(() => {
       if (!currentOS) return null
+      if (!selectedPlatform) return null
       return downloads[`JamClient/${selectedPlatform}`]
-    }, [currentOS]);
+    }, [currentOS, selectedPlatform]);
 
   const selectPlatform = (platform) => {
     setSelectedPlatform(platform)