[Bug]: alternating words with an empty option throws an exception

[rubberbaron]

Is there an existing issue for this?

• [X] I have searched the existing issues and checked the recent builds/commits

What happened?

Using an empty option in an alternation, like "[red|] apple", throws an exception. Easy workaround is to put a space in the empty option, like "[red| ] apple", and then it doesn't error out.

Steps to reproduce the problem

1. type "[red|] apple" as the prompt
2. Press generate

What should have happened?

does not throw an exception

Commit where the problem happens

d5ce044b

What platforms do you use to access the UI ?

Windows

What browsers do you use to access the UI ?

Mozilla Firefox

Command Line Arguments

xformers

List of extensions

wildcards, controlnet

Console logs

Error completing request
Arguments: ('task(wz2r40k8hiugpjz)', '[|red] thing', '\n', [], 70, 0, True, False, 1, 1, 7, -1.0, -1.0, 0, 0, 0, False, 400, 700, True, 0.15, 2, '4x_foolhardy_Remacri', 20, 0, 0, 0, False, 'canny', 'control_sd15_canny(fef5e48e)', 0.1, None, False, 'Scale to Fit (Inner Fit)', False, False, False, False, False, False, '', '', '', 1, '', 0, '', 0, '', True, False, False, False) {}
Traceback (most recent call last):
  File "D:\ai\sdweb\modules\prompt_parser.py", line 76, in alternate
    yield next(args[(step - 1)%len(args)])
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "D:\ai\sdweb\venv\lib\site-packages\lark\visitors.py", line 116, in _call_userfunc
    return f(children)
  File "D:\ai\sdweb\modules\prompt_parser.py", line 84, in start
    return ''.join(flatten(args))
  File "D:\ai\sdweb\modules\prompt_parser.py", line 83, in flatten
    yield from flatten(gen)
  File "D:\ai\sdweb\modules\prompt_parser.py", line 83, in flatten
    yield from flatten(gen)
  File "D:\ai\sdweb\modules\prompt_parser.py", line 82, in flatten
    for gen in x:
RuntimeError: generator raised StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:\ai\sdweb\modules\call_queue.py", line 56, in f
    res = list(func(*args, **kwargs))
  File "D:\ai\sdweb\modules\call_queue.py", line 37, in f
    res = func(*args, **kwargs)
  File "D:\ai\sdweb\modules\txt2img.py", line 52, in txt2img
    processed = process_images(p)
  File "D:\ai\sdweb\modules\processing.py", line 476, in process_images
    res = process_images_inner(p)
  File "D:\ai\sdweb\modules\processing.py", line 604, in process_images_inner
    c = get_conds_with_caching(prompt_parser.get_multicond_learned_conditioning, prompts, p.steps, cached_c)
  File "D:\ai\sdweb\modules\processing.py", line 562, in get_conds_with_caching
    cache[1] = function(shared.sd_model, required_prompts, steps)
  File "D:\ai\sdweb\modules\prompt_parser.py", line 205, in get_multicond_learned_conditioning
    learned_conditioning = get_learned_conditioning(model, prompt_flat_list, steps)
  File "D:\ai\sdweb\scripts\prompt_blending.py", line 40, in hijacked_get_learned_conditioning
    return real_get_learned_conditioning(model, switched_prompts, steps)
  File "D:\ai\sdweb\modules\prompt_parser.py", line 129, in get_learned_conditioning
    prompt_schedules = get_learned_conditioning_prompt_schedules(prompts, steps)
  File "D:\ai\sdweb\modules\prompt_parser.py", line 102, in get_learned_conditioning_prompt_schedules
    promptdict = {prompt: get_schedule(prompt) for prompt in set(prompts)}
  File "D:\ai\sdweb\modules\prompt_parser.py", line 102, in <dictcomp>
    promptdict = {prompt: get_schedule(prompt) for prompt in set(prompts)}
  File "D:\ai\sdweb\modules\prompt_parser.py", line 100, in get_schedule
    return [[t, at_step(t, tree)] for t in collect_steps(steps, tree)]
  File "D:\ai\sdweb\modules\prompt_parser.py", line 100, in <listcomp>
    return [[t, at_step(t, tree)] for t in collect_steps(steps, tree)]
  File "D:\ai\sdweb\modules\prompt_parser.py", line 90, in at_step
    return AtStep().transform(tree)
  File "D:\ai\sdweb\venv\lib\site-packages\lark\visitors.py", line 153, in transform
    return self._transform_tree(tree)
  File "D:\ai\sdweb\venv\lib\site-packages\lark\visitors.py", line 149, in _transform_tree
    return self._call_userfunc(tree, children)
  File "D:\ai\sdweb\venv\lib\site-packages\lark\visitors.py", line 120, in _call_userfunc
    raise VisitError(tree.data, tree, e)
lark.exceptions.VisitError: Error trying to process rule "start":

generator raised StopIteration

Additional information

No response