fix a bug

3rd/box2d/include/box2d_bindings.hpp

@@ -73,7 +73,9 @@ struct PyBody{
     PyBody(): body(nullptr), fixture(nullptr), node_like(nullptr){}
 
     void _gc_mark() {
-        PK_OBJ_MARK(node_like);
+        if(node_like != nullptr){
+            PK_OBJ_MARK(node_like);
+        }
     }
 
     PyBody& _() { return *this; }

3rd/box2d/src/box2d_Body.cpp

@@ -128,7 +128,7 @@ void PyBody::_register(VM* vm, PyObject* mod, PyObject* type){
 
     // destroy
     vm->bind(type, "destroy(self)", [](VM* vm, ArgsView args){
-        PyBody& body = CAST(PyBody&, args[1]);
+        PyBody& body = CAST(PyBody&, args[0]);
         body.body->GetWorld()->DestroyBody(body.body);
         body.body = nullptr;
         body.fixture = nullptr;