Runtime error while trying to zoom in/out the graph.

[sarthakvengurlekar]

To Reproduce
Steps to reproduce the behavior:

1. Run npm start
2. Try to zoom in or out at the graphpage
3. See runtime error

Versions:
"d3-drag": "^1.2.5",
"d3-force": "^2.0.1",
"d3-selection": "^2.0.0",
"d3-shape": "^1.3.7",
"d3-zoom": "^1.8.3",