**RESOLVED** Trying to create a Document reference from a string

I am trying to get a document reference from a string. I have the following:

www.site.com/page?foo=bar
When I go to the page, I want my OnLoad action to get the data from 'collectionName/bar' so I need to convert this 'collectionName/ + bar' into a document reference.

I am currently doing the following:

DocumentReference? convertStringToDocumentReference(String docID) {
  /// MODIFY CODE ONLY BELOW THIS LINE

  final theStr = 'collecionName/' + docID;
  final theRef = theStr as DocumentReference;

  return theRef;

  /// MODIFY CODE ONLY ABOVE THIS LINE
}

But I get the following error.
TypeError: "collectionName/bar": type 'JSString' is not a subtype of type 'DocumentReference'

I've tried this as well

DocumentReference? convertStringToDocumentReference(String docID) {
  /// MODIFY CODE ONLY BELOW THIS LINE

  return FirebaseFirestore.instance.collection('collectionName').doc(docID);

  /// MODIFY CODE ONLY ABOVE THIS LINE
}

But then I get the error:
Error: Undefined name 'FirebaseFirestore'.
return FirebaseFirestore.instance.collection('collectionName').doc(docID);