<p><a href="https://github.com/openstreetmap/openstreetmap-website/blob/36817f777c502a5a044e82fc3e3ca55f5433d66f/app/controllers/users_controller.rb#L452">https://github.com/openstreetmap/openstreetmap-website/blob/36817f777c502a5a044e82fc3e3ca55f5433d66f/app/controllers/users_controller.rb#L452</a></p>
<p>Reproduce in /users page<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/42279873/55848376-4a8ffc80-5b12-11e9-803a-350a7462b79c.png"><img src="https://user-images.githubusercontent.com/42279873/55848376-4a8ffc80-5b12-11e9-803a-350a7462b79c.png" alt="image" style="max-width:100%;"></a></p>
<p>When I don't select any user and click "Confirmed Selected User" button, I will get an error saying params[:user] is nil as bellows.</p>
<p><a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/42279873/55848362-406dfe00-5b12-11e9-86af-92d117cee51b.png"><img src="https://user-images.githubusercontent.com/42279873/55848362-406dfe00-5b12-11e9-86af-92d117cee51b.png" alt="image" style="max-width:100%;"></a></p>
<p>Better error handler should be provided here, such as enforce at least choose one user.</p>

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