Avoid two NPEs which were currently "expected" in tests

When things go wrong, we always want to give an
explanation via an exception instead of an NPE.

poi/src/main/java/org/apache/poi/hssf/record/aggregates/FormulaRecordAggregate.java

@@ -39,7 +39,7 @@ import org.apache.poi.util.RecordFormatException;
 public final class FormulaRecordAggregate extends RecordAggregate implements CellValueRecordInterface {
 
     private final FormulaRecord _formulaRecord;
-    private SharedValueManager _sharedValueManager;
+    private final SharedValueManager _sharedValueManager;
     /** caches the calculated result of the formula */
     private StringRecord _stringRecord;
     private SharedFormulaRecord _sharedFormulaRecord;
@@ -195,6 +195,9 @@ public final class FormulaRecordAggregate extends RecordAggregate implements Cel
         CellReference expRef = _formulaRecord.getFormula().getExpReference();
         if (expRef != null) {
             ArrayRecord arec = _sharedValueManager.getArrayRecord(expRef.getRow(), expRef.getCol());
+            if (arec == null) {
+                throw new IllegalStateException("Could not get ArrayRecord for cell-reference " + expRef.formatAsString());
+            }
             return arec.getFormulaTokens();
         }
         return _formulaRecord.getParsedExpression();

poi/src/main/java/org/apache/poi/hssf/usermodel/HSSFComment.java

@@ -285,7 +285,11 @@ public class HSSFComment extends HSSFTextbox implements Comment {
         byte [] inSp = getEscherContainer().serialize();
         spContainer.fillFields(inSp, 0, new DefaultEscherRecordFactory());
         ObjRecord obj = (ObjRecord) getObjRecord().cloneViaReserialise();
-        NoteRecord note = (NoteRecord) getNoteRecord().cloneViaReserialise();
+        NoteRecord noteRecord = getNoteRecord();
+        if (noteRecord == null) {
+            throw new IllegalStateException("Could not clone the note record for this comment because it is null");
+        }
+        NoteRecord note = (NoteRecord) noteRecord.cloneViaReserialise();
         return new HSSFComment(spContainer, obj, txo, note);
     }