issue-147: Use member variable instead of trying to subscribe object

[Deborah-Digges]

Fixes #147

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Fixes bug in app.py which tries to index into an object rather than using the variable on the object.