I have solved this myself and will provide the solution below, in case anyone is having the same issue.
To summarize my issue: I did not fully inspect the objects and assumed that when I got a null reference that the entire item was null, not just the parameters I was checking. Stupid mistake that cost me a lot of time.
In case anybody is looking at this down the road, ensure you inspect more thoroughly (or just try using it as a reference!).
Below I included my solution. I simply used WView.GetModelItemFromView() to get the ModelItem (which had seemed to be null at the time) and passed it into the CombStateData ModelItems. I had to create multiple views in our scenario, so I had an array list I generated of desired view names. I looped through those names, added a reference to the end of my ModelItems array, and then created the CombState. The only thing I have not figured out yet is how to set one of the CombStates to "DEFAULT", but I am sure I will figure it out.
See below for a snippet of the solution:
ModelItem itemREP = mdl.GetItemByName(ModelItemType.ITEM_SIMPREP, "DEFAULT REP");
ModelItem itemSTYLE = solid.GetActiveStyleState();
Assembly assembly = (Assembly)mdl;
ModelItem itemEXPLODED = assembly.GetActiveExplodedState();
//Defining reference item list
ModelItems itemList = ModelItems.create();
itemList.insert(0, itemREP);
itemList.insert(1, itemSTYLE);
itemList.insert(2, itemEXPLODED);
//Creating CombStates with the references
//Since this requires a different view reference for each CombState,
//it is done in a for loop for each desired view
WView wView;
for (int i = 0; i <= viewNames.length - 1; i++){
wView = (WView) mdl.GetView(viewNames[i]);
ModelItem itemVIEW = wView.GetModelItemFromView();
if (i > 0) {
itemList.set(3, itemVIEW);
} else {
itemList.insert(3, itemVIEW);
}
CombStateData dataTest = wfcCombState.CombStateData_Create(itemList, CrossSectionClipOption.VIS_OPT_NONE, false); //Exploded state = false
state = solid.CreateCombState(combStateNames[i], dataTest);
if (combStateNames[i].toLowerCase() == "ISO".toLowerCase()){
//Just need to figure out how to set ISO to default now.
}
}