Comm Target not found

[TK-21st]

When switching kernel session, or starting widget with known kernel session, a common error is as follows:

default.js:987 Exception opening new comm
_handleCommOpen @ default.js:987
async function (async)
_handleCommOpen @ default.js:980
_handleMessage @ default.js:1123
(anonymous) @ default.js:94
Promise.then (async)
KernelConnection._onWSMessage @ default.js:91
default.js:99 Error: Object 'FBL-Comm:NeuAny-3-8fbc890b-fffc-4224-82eb-65afd8d8e418' not found in registry
    at default.js:1243
    at new Promise (<anonymous>)
    at Object.loadObject (default.js:1221)
    at KernelConnection._handleCommOpen (default.js:980)
    at KernelConnection._handleMessage (default.js:1123)
    at default.js:94

Reproducing

1. Create NeuAny Instance (NeuAny-12)

>> get_ipython().kernel.comm_manager.comms
{'d11b5847215348db8fed3caf076ca8d6': <ipykernel.comm.comm.Comm at 0x7fcac80ddd50>}
>> comm.comm_id
'd11b5847215348db8fed3caf076ca8d6'
>> comm.target_name
'FBL-Comm:NeuAny-12-e9f084d7-52eb-4a7e-9217-f24be2ee1d60'

2. Create another NeuAny Instance (NeuAny-13)

>> get_ipython().kernel.comm_manager.comms
{'9de245123b62460d93ad8bac68a5f22a': <ipykernel.comm.comm.Comm at 0x7fbd98713f90>}
>> comm.comm_id
'9de245123b62460d93ad8bac68a5f22a'
>> comm.target_name
'FBL-Comm:NeuAny-13-04d84a89-2394-4be8-b4a5-7660a4b225c1'

3. Switch NeuAny-12 to use NeuAny-13's sessionContext

default.js:987 Exception opening new comm
_handleCommOpen @ default.js:987
async function (async)
_handleCommOpen @ default.js:980
_handleMessage @ default.js:1123
(anonymous) @ default.js:94
Promise.then (async)
KernelConnection._onWSMessage @ default.js:91
default.js:99 Error: Object 'FBL-Comm:NeuAny-12-e9f084d7-52eb-4a7e-9217-f24be2ee1d60' not found in registry
    at default.js:1243
    at new Promise (<anonymous>)
    at Object.loadObject (default.js:1221)
    at KernelConnection._handleCommOpen (default.js:980)
    at KernelConnection._handleMessage (default.js:1123)
    at default.js:94

>> get_ipython().kernel.comm_manager.comms
{'9de245123b62460d93ad8bac68a5f22a': <ipykernel.comm.comm.Comm at 0x7fbd98713f90>}
>> [c.comm_id for c in comms.values()]
['9de245123b62460d93ad8bac68a5f22a', '3c9ae041be344ae49ca5f3ec7870361e']
>> [c.target_name for c in comms.values()]
['FBL-Comm:NeuAny-13-04d84a89-2394-4be8-b4a5-7660a4b225c1',
 'FBL-Comm:NeuAny-12-e9f084d7-52eb-4a7e-9217-f24be2ee1d60']

[TK-21st]

Note that this error does not prevent comm from being sent, i.e. after switching kernels we can send the data as expected

list(comms.values())[0].send('test')
comm message received on NeuAny-13
list(comms.values())[0].send('test')
comm message received on NeuAny-12

[TK-21st]

Moving this issue to V3 milestone since it is not currently influence proper functioning of v2.