Dotplot: Load sequence button doesn't work

To reproduce
1. Select "Build dotplot" from main menu
2. Select two sequences and press Next
(sequences are loaded and another dialog appears)
3. Press "Load sequence" button and select a fasta file in appeared dialog
(nothing happens. If cancel pressed, sequence is loaded)

Problem 1: a new task for opening view for selected file is added, but not executed until the dialog is closed.
Problem 2: if selected file was a fasta with multiple sequences (e.g. _common_data/scenarios/dp_view/dpm1.fa), after closing dialog its loading causes crash on safe point:

[ERROR][18:37:25] Trying to recover from error: Item modification not allowed at src/models/StateLockableDataModel.cpp:218