Fix splatfacto crash in eval when using viewer (#3430)

Author: aayushg55

nerfstudio/engine/trainer.py

@@ -300,7 +300,8 @@ def train(self) -> None:
 
                 # Do not perform evaluation if there are no validation images
                 if self.pipeline.datamanager.eval_dataset:
-                    self.eval_iteration(step)
+                    with self.train_lock:
+                        self.eval_iteration(step)
 
                 if step_check(step, self.config.steps_per_save):
                     self.save_checkpoint(step)