fixed issue with dop prompt replacement

extensions_built_in/sd_trainer/SDTrainer.py

@@ -1362,6 +1362,8 @@ class SDTrainer(BaseSDTrainProcess):
                             
                             if self.train_config.diff_output_preservation:
                                 dop_prompts = [p.replace(self.trigger_word, self.train_config.diff_output_preservation_class) for p in conditioned_prompts]
+                                dop_prompts_2 = None
+                                if prompt_2 is not None:
                                     dop_prompts_2 = [p.replace(self.trigger_word, self.train_config.diff_output_preservation_class) for p in prompt_2]
                                 self.diff_output_preservation_embeds = self.sd.encode_prompt(
                                     dop_prompts, dop_prompts_2,