Bug in regex_mutator unicode generation

[z2-2z]

Hello,
it seems like there is a bug in the regex mutator.
If you supply the following grammar to the generator

ctx.rule("START", "{CHAR}")
ctx.regex("CHAR", ".")

it panics every other invocation in append_unicode_range():

$ ./target/release/generator -g ./demo.py -t 1
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', regex_mutator/src/lib.rs:113:45
stack backtrace:
   0: rust_begin_unwind
             at /rustc/9dd4ce80fb01d1ff5cb5002f08b7b3847b59e664/library/std/src/panicking.rs:517:5
   1: core::panicking::panic_fmt
             at /rustc/9dd4ce80fb01d1ff5cb5002f08b7b3847b59e664/library/core/src/panicking.rs:103:14
   2: core::panicking::panic
             at /rustc/9dd4ce80fb01d1ff5cb5002f08b7b3847b59e664/library/core/src/panicking.rs:50:5
   3: regex_mutator::generate
   4: grammartec::rule::Rule::generate
   5: grammartec::tree::Tree::generate_from_rule
   6: grammartec::context::Context::generate_tree_from_nt
   7: generator::main