NullReference on object[] (item) #217 (#218)

(cherry picked from commit 703ce73)

Author: b0bi79

ClosedXML.Report/Options/GroupTag.cs

@@ -101,7 +101,7 @@ public override void Execute(ProcessingContext context)
 
         private bool DisableGrandTotal => List.HasTag("disablegrandtotal") || !List.GetAll<SummaryFuncTag>().Any();
 
-        private void Process(ProcessingContext context, GroupTag[] groups, bool summaryAbove, SummaryFuncTag[] summaries , bool disableGrandTotal)
+        private void Process(ProcessingContext context, GroupTag[] groups, bool summaryAbove, SummaryFuncTag[] summaries, bool disableGrandTotal)
         {
             var root = context.Range;
             var groupRow = root.LastRow();

ClosedXML.Report/Utils/TypeExtensions.cs

@@ -61,7 +61,10 @@ public static Type GetItemType(this IEnumerable sourceList)
 
         public static bool IsAllItemsAssignableFrom(this IEnumerable list, Type type)
         {
-            return list.Cast<object>().All(item => item.GetType().IsAssignableFrom(type));
+            return list.Cast<object>().All(item =>
+                type.IsValueType
+                    ? item != null && item.GetType().IsAssignableFrom(type)
+                    : item == null || item.GetType().IsAssignableFrom(type));
         }
 
         internal static bool IsNumeric(this Type type)