Fix crash when adding shadow casters

[tomas7770]

Description

Adding a shadow caster to an existing light source in the entity inspector crashes the engine with one of the following errors:

Spot/point lights:

terminate called after throwing an instance of 'std::out_of_range'
  what():  map::at
Aborted (core dumped)

Directional lights:

terminate called after throwing an instance of 'std::out_of_range'
  what():  unordered_map::at
Aborted (core dumped)

Reproduce

Steps to reproduce the behavior:

1. Open an engine sample/game
2. Open World Inspector in the Toolbox and select any entity
3. Open Entity Inspector
4. Add a light component if the entity doesn't have one (e.g. DirectionalLight)
5. Add the corresponding shadow caster component (e.g. DirectionalShadowCaster)

Expected behaviour

Adding a shadow caster should enable shadows without crashing.