Fix corruption of skeleton when using CTRL+Rightclick in empty tree

[philippotto]

The bug has two facets:

1. Using ctrl+rightclick to add a tree does not select and also not center it. This means, using ctrl+rightclick in an empty tree twice, will try to create two nodes within the same tree. However, if no node is selected, no edge can be constructed for the second node which produces the corruption.
2. The addNode reducer helper simply added a node when being told so and only created an edge, if a node was selected. This means, if no node is selected (for whatever reason), corruption can ensue when the tree is not-empty. This should not happen in theory, but (1) provoked this scenario.

To fix this bug, I did 2 things:

1. Ensure that ctrl+rightclick also selects the first node of a tree (it's still not centered). I think, this is a fair behavior.
2. I adapted the addNode code so that the corruption cannot happen even if the circumstances would provoke it. The reducer would simply do a noop instead (and log an error message to the console).

URL of deployed dev instance (used for testing):

• https://fixcorrupttrees.webknossos.xyz

Steps to test:

• Create/Activate an empty tree
• Use Ctrl + Rightclick to create a first node
• Use Ctrl + Rightclick to create a second node

Issues:

• fixes Ctrl + Rightclick causes skeleton annotation corruption #5303

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• Updated (unreleased) changelog
• Updated (unreleased) migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Needs datastore update after deployment
• Ready for review

[daniel-wer]

Very nice solution! 👍