Check that there are no pending keyframes while writing a spec.

This can happen when a body is detached without compiling or recompiling.

Also added error catching when trying to delete a body instead of detaching it.

Fixes #2327

PiperOrigin-RevId: 712526730
Change-Id: I4b48df83120fca12d475c85b3a54893841449653

src/xml/xml_native_writer.cc

@@ -858,7 +858,7 @@ mjXWriter::mjXWriter(void) {
 // cast model
 void mjXWriter::SetModel(const mjSpec* _spec, const mjModel* m) {
   if (_spec) {
-    model = (mjCModel*)_spec->element;
+    model = static_cast<mjCModel*>(_spec->element);
   }
   if (m) {
     model->CopyBack(m);
@@ -2207,6 +2207,10 @@ void mjXWriter::Keyframe(XMLElement* root) {
   // create section
   XMLElement* section = InsertEnd(root, "keyframe");
 
+  if (!model->key_pending_.empty()) {
+    throw mjXError(0, "Model has pending keyframes. It must be (re)compiled before writing XML.");
+  }
+
   // write all keyframes
   for (int i=0; i<model->nkey; i++) {
     XMLElement* elem = InsertEnd(section, "key");