### Godot version

4.1.3 stable.arch_linux

### System information

Artix Linux Godot v4.1.3 stable.arch_linux

### Issue description

![2023-11-14T16:29:28,788232528+01:00](https://github.com/godotengine/godot/assets/92468390/a79027e2-1539-4ca9-9369-978c32dea26d)
Both "Target Surface:" and "Source Mesh:" accept non Node3D nodes which doesn't really make any sense as they need 3D geometry to function.

### Steps to reproduce

1. Add a MultiMeshInstance3D
2. Add a Node that doesn't inherit Node3D
3. Open the "Populate Surface" menu and click the right button of either "Target Surface:" or "Source Mesh:"

### Minimal reproduction project

N/A
#include "multimesh_editor_plugin.h"
#include "editor/editor_node.h"
#include "editor/editor_string_names.h"
#include "editor/gui/scene_tree_editor.h"
#include "editor/plugins/node_3d_editor_plugin.h"
#include "scene/3d/mesh_instance_3d.h"
#include "scene/gui/box_container.h"
#include "scene/gui/menu_button.h"
#include "scene/gui/option_button.h"
void MultiMeshEditor::_node_removed(Node *p_node) {
	if (p_node == node) {
		node = nullptr;
		hide();
	}
}
void MultiMeshEditor::_populate() {
	if (!node) {
		return;
	}
	Ref<Mesh> mesh;
	if (mesh_source->get_text().is_empty()) {
		Ref<MultiMesh> multimesh;
		multimesh = node->get_multimesh();
		if (multimesh.is_null()) {
			err_dialog->set_text(TTR("No mesh source specified (and no MultiMesh set in node)."));
			err_dialog->popup_centered();
			return;