Crash when attempting to generate a configuration using a species without a forcefield

[aboswel]

Description
A crash occurs when finishing the new configuration wizard if the included species does not have a forcefield. A "Segmentation fault (core dumped)" error is provided.

To Reproduce
Steps to reproduce the behaviour:

1. Open Dissolve and create a new file
2. Create a new atomic species
3. Create a new configuration of said species with default settings
4. Experience crash

Expected behaviour
Configuration wizard checks the species' validity before generation, prevents generation if invalid.

Screenshots
gdb backtrace:

Operating System:

• OS: Linux
• OS version / flavour: IDAaaS Developer