ASSERT when switching to map with object selected

The following assert happens when tabbing between maps when an object is selected.

ASSERT: "layer->isObjectGroup() || layer->isGroupLayer()" in file /home/user/tiled/src/tiled/mapobjectmodel.cpp, line 313

**To Reproduce**
Steps to reproduce the behavior:
1. Open Tiled with two maps loaded (from previous session).
2. Click on an object with the 'S' select tool.
3. Click on the other map tab, then click on the first tab again.
4. ASSERT occurs

**Specifications:**
- OS: Linux
- Tiled Version: Built from commit c48134342 (v1.9.2, Qt 5.15.2)

Start by reading tiled/mapobjectmodel.cpp around line 313 and reproduce the assertion with two maps loaded and an object selected. Trace the tab-switching path, then verify that switching between map tabs with an object selected no longer triggers the assertion.

Written by the indexing model from the issue text.