gfs261091
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In the properties of a map created with these settings, find the "objectives" tab => "primary" => "common" => [1] ([0] means "defeat all"). Go to the properties of task [1] by clicking "+", and find the "ignore" line; set it to "true".
I think I found another solution – I chose "Defeat all" instead of building the Grail. If that doesn't work, I'll try your suggestion.