Crash on null pointer access, when SSP import fails

[rruusu]

Description

When an SSP import fails, OMSimulator crashes on a segmentation fault due to null pointer access.

Steps to reproduce the behavior

Add an unsupported element into model description. Import the SSP.

Expected behavior

OMSimulator should report the error and not crash.

[arun3688]

@rruusu Can you create an example and i see that you are are making changes to the code base it will be good if you can create a PR with your changes and we can merge it. The PR's can be small so that it is easy to keep track