fix: Add None-check for Logic-Node condition/expression

Previous fix handled hasattr() but didn't check for None values.
Now explicitly checks that operator/expression is not None before using it.

Error was: "'NoneType' object has no attribute 'operator'"

Clearer error message: "condition is None or missing"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/workflow_executor.py

@@ -412,12 +412,18 @@ def execute_logic_node(
         # Handle both serialization formats:
         # UI format: condition = {operands: [...], operator: "and"}
         # Expected format: condition = {expression: {operands: [...], operator: "and"}}
+        expression = None
+
         if hasattr(node.condition, 'operator') and hasattr(node.condition, 'operands'):
-            expression = node.condition  # UI format (direct)
-        elif hasattr(node.condition, 'expression'):
-            expression = node.condition.expression  # Expected format (wrapped)
-        else:
-            error_msg = f"Logic node {node.id} has invalid condition structure (missing operator/operands)"
+            # UI format (direct) - check that operator is not None
+            if node.condition.operator is not None:
+                expression = node.condition
+        elif hasattr(node.condition, 'expression') and node.condition.expression is not None:
+            # Expected format (wrapped) - check that expression is not None
+            expression = node.condition.expression
+
+        if expression is None:
+            error_msg = f"Logic node {node.id} has invalid or empty condition (operator/operands/expression is None or missing)"
             logger.error(error_msg)
             return NodeExecutionState(
                 node_id=node.id,