Graph adjacency matrix not being cleaned up properly

[JMCanning78]

Steps to recreate the problem:

1. Launch the Graph.py visualization
2. Randomly fill with 5 vertices
3. Select New Graph
4. Randomly fill with 2 vertices

This leaves a red panel in the adjacency matrix like this:

There appears to be some leftover widgets in the adjacency matrix window that are not being cleaned up when the graph is emptied. The width of red panel grows with the previous max number of vertices.