TypeError: Cannot read property 'unobserve' of null
   at ExternalComponent../node_modules/@snunit/cm-viewer-2/src/client2/cm/node/items/external/external.component.ts.ExternalComponent.ngOnDestroy (/nodejs/cm/environments/btc-snunit/dist/server.js:346610:23)
   at callProviderLifecycles (/nodejs/cm/environments/btc-snunit/dist/server.js:258209:18)
   at callElementProvidersLifecycles (/nodejs/cm/environments/btc-snunit/dist/server.js:258177:13)
   at callLifecycleHooksChildrenFirst (/nodejs/cm/environments/btc-snunit/dist/server.js:258167:29)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259229:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execEmbeddedViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259298:17)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259227:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execComponentViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259277:13)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259228:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execEmbeddedViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259298:17)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259227:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execComponentViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259277:13)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259228:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execEmbeddedViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259298:17)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259227:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execComponentViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259277:13)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259228:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execEmbeddedViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259298:17)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259227:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execComponentViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259277:13)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259228:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execEmbeddedViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259298:17)
   at destroyView (/nodejs/cm/environments/btc-snunit/dist/server.js:259227:5)
   at callViewAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259355:13)
   at execComponentViewsAction (/nodejs/cm/environments/btc-snunit/dist/server.js:259277:13)