yes

Yes.

yes they are of same size.

Empty arrays or null/nil values being passed into the method should result in an empty string being returned.

You're trying to read the length of null as the error says.

Not a kata issue. Your solution fails some valid test cases, for example this one: assert.equal(wordPattern("heLlo"), "0.1.2.2.3");. I don't think your solution is working in other environment, I think you did not test it sufficiently.

Sonds like a problem on your end - read this. If this were to be a kata issue (i.e the kata is broken) you would need to show how the kata is broken not how your code is broken

It should find two different items in the array

The same index is the same item.

null has no length, read maynardz's post below, it's been already asked and answered.