Steps to repro:

1. Open attached project;
2. In Project panel drag 't1' scene on top of 't2';
3. Click 'Merge into existing';
4. Click 'Semantic and merge tool'.

Actual result:
Popup with text: 'Finish merging (and saving) mergeresult before pressing Accept' is shown. See attached screenshot.

Expected result:
Popup with text: 'Finish merging (and saving) merge result before pressing Accept' is shown.

Notes:
Reproducible in 2017.3.0a6, 2017.2.0b10, 2017.1.0p4.