prevent exception generating tooltip for deleted nodes

[Steve-Mcl]

fixes #3740 (Uncaught TypeError error in browser console when deleting nodes from canvas)

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Check that node exists inside the timer callback - before attempting to generate a tooltip and calculate offsets etc.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.006%) to 68.322% when pulling 9626725 on Steve-Mcl:fix-tooltip-exception into 8cb212d on node-red:master.