fix sampler fallback

modules/processing_helpers.py

@@ -530,6 +530,8 @@ def update_sampler(p, sd_model, second_pass=False):
             else:
                 shared.opts.schedulers_use_karras = False
         sampler = sd_samplers.create_sampler(sampler.name, sd_model)
+        if sampler is None:
+            return
         sampler_options = []
         if sampler.config.get('use_karras_sigmas', False):
             sampler_options.append('karras')

modules/sd_samplers.py

@@ -54,9 +54,8 @@ def create_sampler(name, model):
         shared.log.debug(f'Sampler default {type(model.scheduler).__name__}: {config}')
         return model.scheduler
     config = find_sampler_config(name)
-    if config is None:
-        shared.log.error(f'Attempting to use unknown sampler: {name}')
-        config = all_samplers[0]
+    if config is None or config.constructor is None:
+        return None
     if shared.backend == shared.Backend.ORIGINAL:
         sampler = config.constructor(model)
         sampler.config = config