Eliminates minor javascript error in toolbar popup button

#### Whenever a modal is render through a toolbar popup button, an error is logged in the browser.

This is due to `onClick` been set as Url.

This PR changes `onclick` to value (acceptable option for buttons)

#### Testing
Apply #5386
There are two javascript errors logged in your browsers console (the second one `Joomla.toggleSidebar` is solved #5259)
Apply this PR
error should go away