Script Generator: Errors when setting cell focus

After some combinations of manipulating cells in the script generator table using add, duplicate, delete, clear etc. a null argument is passed when setting the cell focus. This causes an error in the logs though doesn't seem to have an effect on the script generator itself. As a maintainer of the script generator, I would like for the script generator to not be incorrectly setting cell focus.

Error:
```
!ENTRY org.eclipse.ui 4 0 2020-06-22 11:18:46.432
!MESSAGE Unhandled event loop exception
!STACK 0
org.eclipse.core.runtime.AssertionFailedException: null argument:
at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:88)
at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:76)
at org.eclipse.jface.viewers.StructuredSelection.(StructuredSelection.java:82)
at org.eclipse.jface.viewers.ColumnViewer.editElement(ColumnViewer.java:418)
at uk.ac.stfc.isis.ibex.ui.scriptgenerator.views.ActionsViewTable.setCellFocus(ActionsViewTable.java:194)
at uk.ac.stfc.isis.ibex.ui.scriptgenerator.views.ActionsViewTable.setRows(ActionsViewTable.java:183)
at uk.ac.stfc.isis.ibex.ui.scriptgenerator.views.ScriptGeneratorViewModel.lambda$14(ScriptGeneratorViewModel.java:456)
```