When iterating over the layers in a Grease Pencil object,
removing the layers can lead to crashes.
This PR is a better alternative to !135253.
Instead of checking if the layer is valid, simply fetch the latest layer
cache for each iteration and check if the index is valid.
This means that we will skip over layers if they get removed during
iteration, but we no longer can crash. If users want to remove multiple
layers at once, it's better to do this in two passes. E.g. by first
collecting the names of the layers to remove, iterating over
these names and then removing the layers by getting it by its name.
Pull Request: https://projects.blender.org/blender/blender/pulls/135574
5f464b2de0