Here is what I would do to debug and find the root cause:

 - Open the page in Firefox (suggestion, it can be any browser with a JS console)
 - Open the Console
 - Manually call the function in the Console:

`SP.ListOperation.Selection.getSelectedItems(SP.ClientContext.get_current())`

- Check if any value is returned
- If there is a value, then the problem relies in other parts of your function
- Continue by typing `redirectToDownloadsPage` (no parenthesis, just to see if the function exists)
- Is the function registered on the client side?
- If it is, call it by doing `redirectToDownloadsPage()`
- Does it work now?