fix plotShape returning empty figure

[rodriguez-facundo]

No description provided.

[rodriguez-facundo]

plotShape in 2nd tab shows an empty plot.

Reproduce:

• create a simple network from tut3 in test/examples
• go second tab
• click plotShape. ---> figure will be empty

---

Methods getAvailableCellModels and getAvailableCellTypes

These methods will fail if the pops have no cellModel or cellType because they are trying to assign to cm and ct the value of a key that does not exist.

Reproduce:

• enter in Jupiter notebook the following: netpyne_geppetto.netParams.popParams['E'] = {'cellType': 'PT', 'numCells': 3}
• open cellRule tab
• create a new cellRule. ----> the warning will appear.

Really minor coding fixing so, attaching 2 PR together