Stop accessing possibly `undefined` or assume array length without any checks

[FranjoMindek]

Turning on strict setting in tsc showed we had many assumptions about data which may not hold true.
A lot of code accesses possibly undefined objects, or assumes arrays are of certain size without any checks.
We should add proper checks before accessing possibly undefined data.

[Martinsos]

Or would be even better if we can possibly ensure at type level that they are not undefined (either by doing the validation early, or because we actually know so from the start).

[FranjoMindek]

That's the goal. Ensure they are not undefined so we can access them freely.