Can't duplicate pages in Canvas

[cqliu1]

Kibana version: master (commit e5fa6e1)

Elasticsearch version: 7.0.0 snapshot

Server OS version: MacOS 10.14.2

Browser version: Google Chrome Version 71.0.3578.98

Browser OS version: MacOS 10.14.2

Describe the bug:

Page cloning stopped working. When you try to duplicate a page in a Canvas workpad, it crashes Kibana.

Steps to reproduce:

1. Open page manager
2. Hover over a page
3. Click the duplicate button

Expected behavior:

Page should be duplicated.

Screenshots (if relevant):

Errors in browser console (if relevant):

window_error_handler.js:75 TypeError: Cannot read property 'id' of undefined
    at clone_subgraphs.js:20
    at Array.map (<anonymous>)
    at cloneSubgraphs (clone_subgraphs.js:19)
    at clonePage (pages.js:61)
    at pages.js:99
    at eval (handleAction.js:70)
    at eval (index.js:13)
    at Array.reduce (<anonymous>)
    at eval (index.js:12)
    at eval (handleActions.js:48)
    at eval (index.js:32)
    at Array.reduce (<anonymous>)
    at eval (index.js:31)
    at combination (redux.js:441)
    at eval (index.js:32)
    at Array.reduce (<anonymous>)

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-canvas

[monfera]

Thanks Catherine, fixed in #27581
Note: the issue happens with the templates and potentially, preexisting workpads, but doesn't happen with new workpads.