Fix early_stop_condition setting if None

pysr/sr.py

@@ -1311,7 +1311,9 @@ class PySRRegressor(MultiOutputMixin, RegressorMixin, BaseEstimator):
             complexity_of_operators = Main.eval(complexity_of_operators_str)
 
         custom_loss = Main.eval(self.loss)
-        early_stop_condition = Main.eval(str(self.early_stop_condition))
+        early_stop_condition = Main.eval(
+            str(self.early_stop_condition) if self.early_stop_condition else None
+        )
 
         mutation_weights = np.array(
             [