[vscode] hosted-plugin error thrown when closing frontend instance

[vince-fugnitto]

Description

An uncaught error is thrown when a Theia instance is closed with a plugin present (due to the output channel):

root ERROR [hosted-plugin: 12222] Promise rejection not handled in one second: Error: Unknown actor OutputChannelRegistryMain , reason: Error: Unknown actor OutputChannelRegistryMain

root ERROR [hosted-plugin: 12222] With stack trace: Error: Unknown actor OutputChannelRegistryMain\n
    at RPCProtocolImpl.doInvokeHandler (/home/evinfug/theia-workspace/theia/packages/plugin-ext/lib/common/rpc-protocol.js:249:19)\n
    at RPCProtocolImpl.invokeHandler (/home/evinfug/theia-workspace/theia/packages/plugin-ext/lib/common/rpc-protocol.js:240:41)\n
    at RPCProtocolImpl.receiveRequest (/home/evinfug/theia-workspace/theia/packages/plugin-ext/lib/common/rpc-protocol.js:204:31)\n
    at RPCProtocolImpl.receiveOneMessage (/home/evinfug/theia-workspace/theia/packages/plugin-ext/lib/common/rpc-protocol.js:173:22)\n
    at /home/evinfug/theia-workspace/theia/packages/plugin-ext/lib/common/rpc-protocol.js:64:109\n
    at /home/evinfug/theia-workspace/theia/packages/plugin-ext/lib/common/rpc-protocol.js:280:17\n
    at /home/evinfug/theia-workspace/theia/packages/core/lib/common/event.js:162:33\n
    at CallbackList.invoke (/home/evinfug/theia-workspace/theia/packages/core/lib/common/event.js:177:39)\n
    at Emitter.fire (/home/evinfug/theia-workspace/theia/packages/core/lib/common/event.js:265:29)\n
    at ChildProcess.<anonymous> (/home/evinfug/theia-workspace/theia/packages/plugin-ext/lib/hosted/node/hosted-plugin-process.js:121:21)

How to test:

1. start the application using a plugin (ex: @theia/vscode-builtin-git)
2. close the browser instance
3. notice the logs on the backend

[akosyakov]

A dup of #5839?

[RomanNikitenko]

Something similar to #5019