GUI: Trying to delete a single item in sample editor deletes whole configuration

Expected behavior

By pressing the "remove" button only the selected item shall be deleted

Actual behavior

By pressing the "remove" button the whole configuration is deleted

Steps to reproduce the behavior

Go to sample editor

Insert standard sample "Cylinder and prisms"

In the hierarchical tree select "Particle1" (as expected it gets also selected in the graphical editor)

Press the remove button of the editor

Now the complete configuration is gone

Platforms affected

All platforms; BornAgain version 1.18.0

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Original issue: Guthub#1126