Verify elements are non-null before adding event listener

[galactic-src]

When loading www.freecen.org.uk, I see a few console errors. Two of these are Uncaught TypeError: Cannot read properties of null (reading 'addEventListener').

This change checks the elements having click listeners attached are truthy before reading their addEventListener (or attachEvent) attributes. It also future-proofs the subsequent check for element #trigger-offcanvas-old.