When the script is not marked as `@tool` the static constructor is not
called and thus the variables contain `null` by default. But since some
validated operations requires a valid value, this would cause a crash.
This commit solves this by initializing the static variables with a
default value based on their types in the editor, when they are not
marked as `@tool`, so if some `@tool` script access them, they will have
a valid typed value, avoiding the crash.