There are few possibilities:

you have already inserted null under the mykey before and now you are getting it back since if statement returns true
myval is null and your code works flawlessly
mykey is of type which has a very strange implementation of GetHashCode
The first 2 options are not actually possible, as hashtbl[mykey].ToString() would throw NullObjectReference. I would also assume that this is not the real code as I can't think of a scenario where x.ToString() would return null, unless you forgot to mention that myval is of your custom type which overrides it in a strange way.

Therefore, I would assume the latter: myval is your type which overrides ToString in a way that it returns null.

EDIT: after showing your code and clarifying that the last line doesn't return null but instead throws, it looks like the second scenario is happening. If you run your code through debugger, you will see that you are inserting null values for each entry where key index is > 2. Therefore, hashtable returns null.

This snippet is suspicious: ValueArray.GetUpperBound(0) - 1. Try removing '-1'.