Installer hangs when changing language selection

**Describe the bug**
Language selection hangs when user clicks `←Languages` button in submenu.

This is shown in console while it hangs:
```
(io.elementary.installer:4164): Gtk-CRITICAL **: 23:35:14.760: gtk_widget_draw: assertion '!widget->priv->alloc_needed' failed
```

This issue does not occur in initial-setup, only in installer.

**To Reproduce**
Steps to reproduce the behavior:
1. When choosing language click `English`, then click `←Languages`
2. Setup hangs until some other window takes focus (e.g. wingpanel)

**Expected behavior**
Come back to language selection menu without hanging, like in initial-setup.

**Desktop (please complete the following information):**
elementary OS 6 beta 2021.04.30