bound* causes error in certain circumstances

[onlyafly]

I discovered this error in my own application, but I have reproduced it at the Widje example at https://github.com/lynaghk/todoFRP/tree/master/todo/widje

Steps to reproduce:

1. Launch the widje To Do application.
2. Create three todo items called "a", "b", and "c"
3. Delete "c".
4. Attempt to edit "b"
5. Notice the JavaScript error "No protocol method IDeref.-deref defined for type null:"

Notes:

This seems to be caused by line 62 of widje/core.cljs:

(-value [this] (apply value-func (map deref atoms)))

When one of the atoms used in a bound* is a SubAtom produced using bound-coll, it can sometimes attempt to deref a nil atom, leading to the deref error. This seems to only occur after removing an element in the bound collection.

[onlyafly]

I have tracked the problem down to the source: crate.binding/sub-destroy! removes all watches from the sub atom and sets its parent atom to nil. However, when using bound_, a single watch function can point to two atoms, which means the other atom in bound_ maintains the watch.