[plugin] Cannot read property 'subscriptions' of undefined

[akosyakov]

I can reproduce it with https://github.com/sstone1/che-plugin-registry/releases/download/0.0.1/ibm-blockchain-platform-1.0.3.vsix

root ERROR [hosted-plugin: 1970] (node:1970) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'subscriptions' of undefined
    at disposeExtension (/tmp/vscode-unpacked/ibm-blockchain-platform-1.0.3.vsix/extension/out/src/extension.js:319:13)
    at ActivatedPlugin.<anonymous> (/tmp/vscode-unpacked/ibm-blockchain-platform-1.0.3.vsix/extension/out/src/extension.js:209:9)
    at Generator.next (<anonymous>)
    at fulfilled (/tmp/vscode-unpacked/ibm-blockchain-platform-1.0.3.vsix/extension/out/src/extension.js:17:58)
    at process._tickCallback (internal/process/next_tick.js:68:7)

It seems to happen when the window is closed.