Krita crashes when layer group uses "pass-through" mode and "All Layers" is pressed in Batch Exporter.

**I'm submitting a...**

- [x] Bug report.
- [ ] Feature request.

## Bug report ##

Krita crashes when layer group uses "pass-through" mode and "All Layers" is pressed in Batch Exporter.

**What is the current behavior?**

Krita crashes when layer group uses "pass-through" mode and "All Layers" is pressed in Batch Exporter.

**What is the expected behavior?**

Krita batch-exports the layer group correctly.

**Tell us the steps to reproduce the bug, and if possible share a minimal demo of the problem.**

1. Open file https://www.dropbox.com/s/ixkq5awaw8ao6tr/firetex.kra?dl=0
2. If not enabled yet, enable the "pass trough" mode for the layer group with the name "firetex_d e=tga"
3. Press "All layers" in the Batch Exporter
4. Krita Crashes

Start by reproducing the crash with the linked firetex.kra file: enable pass-through on the named layer group and press "All layers" in the Batch Exporter. Trace the batch export path involved in processing layer groups; done means the group exports correctly without Krita crashing.

Written by the indexing model from the issue text.