Fix #934: segfault when substituting a SubTree node (#1083)

* Fix #934: prevent segfault when substituting a SubTree node

When a substitution rule replaced a SubTree with a TestNode,
dynamic_cast<SubTreeNode*> returned nullptr but setSubtreeID was
called unconditionally, causing a null pointer dereference. Add a
null-check so setSubtreeID is only called on actual SubTree nodes.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix clang-tidy readability-implicit-bool-conversion warnings

Use explicit != nullptr comparisons for pointer-to-bool conversions
flagged by clang-tidy.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide

src/xml_parsing.cpp

@@ -867,8 +867,13 @@ TreeNode::Ptr XMLParser::PImpl::createNodeFromXML(const XMLElement* element,
     config.input_ports = port_remap;
     new_node =
         factory->instantiateTreeNode(instance_name, toStr(NodeType::SUBTREE), config);
+    // If a substitution rule replaced the SubTree with a different node
+    // (e.g. a TestNode), the dynamic_cast will return nullptr.
     auto subtree_node = dynamic_cast<SubTreeNode*>(new_node.get());
-    subtree_node->setSubtreeID(type_ID);
+    if(subtree_node != nullptr)
+    {
+      subtree_node->setSubtreeID(type_ID);
+    }
   }
   else
   {

tests/gtest_substitution.cpp

@@ -50,3 +50,39 @@ TEST(Substitution, Parser)
 
   ASSERT_EQ(*std::get_if<std::string>(&rules.at("actionC")), "NotAConfig");
 }
+
+// Regression test for issue #934: segfault when substituting a SubTree node
+TEST(Substitution, SubTreeNodeSubstitution)
+{
+  static const char* parent_xml = R"(
+  <root BTCPP_format="4">
+    <BehaviorTree ID="Parent">
+      <SubTree ID="Child" name="child" />
+    </BehaviorTree>
+  </root>
+  )";
+
+  static const char* child_xml = R"(
+  <root BTCPP_format="4">
+    <BehaviorTree ID="Child">
+      <AlwaysSuccess />
+    </BehaviorTree>
+  </root>
+  )";
+
+  BehaviorTreeFactory factory;
+  factory.registerBehaviorTreeFromText(parent_xml);
+  factory.registerBehaviorTreeFromText(child_xml);
+
+  BT::TestNodeConfig config;
+  config.return_status = BT::NodeStatus::SUCCESS;
+  factory.addSubstitutionRule("child", config);
+
+  // This should not crash (was a segfault before the fix)
+  Tree tree;
+  ASSERT_NO_THROW(tree = factory.createTree("Parent"));
+
+  // The substituted tree should tick successfully
+  auto status = tree.tickWhileRunning();
+  ASSERT_EQ(status, BT::NodeStatus::SUCCESS);
+}