AlignmentViewer hangs for empty/single-sequence MSAs (instead of throwing an error)

[BernhoferM]

When uploading an empty MSA or one that contains only a single sequence, the website just stops during the loading/rendering process instead of providing any error/warning message. While such a MSA does not make much sense, this case can occur when the MSA and links to AlignmentViewer are generated programmatically.

In case of an empty alignment, the website just hangs at "reading MSA...", while a single-sequence MSA hangs at "rendering MSA .. 0%."

[sacdallago]

@rocreguant could you look into this :) relevant for the PP paper :)

Here's an example:

https://alignmentviewer.org/?url=https%3A%2F%2Fpredictprotein.org%2Fapi%2Fppc_fetch%3Faction%3Dget%26hash%3Dad43ddd0d824e094a0d5b66a3594961a109c83a0%26file%3Dquery.mmseqs2AliUref

[cannin]

@BernhoferM thanks for the comment. we had a quick discussion. i have made a Troubleshooting section on the README that mentions this issue and tries to guide users if they see the problem. for the time being, i'll label this wontfix unless we see more users facing this issue since it seems like an edge case. if you have time feel free to submit a quick pull request to address the issue.