Suggested fix for NullReferenceException in C#

csharp/DiffMatchPatch.cs

@@ -69,6 +69,9 @@ public class Diff {
     public Diff(Operation operation, string text) {
       // Construct a diff with the specified operation and text.
       this.operation = operation;
+      if (text == null) {
+        throw new NullReferenceException("text must contain a value.");
+      }
       this.text = text;
     }
 

csharp/tests/DiffMatchPatchTest.cs

@@ -1095,6 +1095,20 @@ public void patch_applyTest() {
     assertEquals("patch_apply: Edge partial match.", "x123\tTrue", resultStr);
   }
 
+  public void diff_DiffNullTextTest() {
+    var operations = Enum.GetValues<Operation>();
+    foreach (var operation in operations) {
+      try {
+        var diff = new Diff(operation, null);
+        assertFail($"diff_DiffNullTextTest: operation {operation} with null text.");
+      } catch (NullReferenceException) {
+        // Exception expected.
+      } catch {
+        assertFail($"diff_DiffNullTextTest: operation {operation} with null text throwed an invalid exception.");
+      }
+    }
+  }
+
   private string[] diff_rebuildtexts(List<Diff> diffs) {
     string[] text = { "", "" };
     foreach (Diff myDiff in diffs) {
@@ -1225,6 +1239,8 @@ public static void Main(string[] args) {
     dmp.patch_addPaddingTest();
     dmp.patch_applyTest();
 
+    dmp.diff_DiffNullTextTest();
+
     Console.WriteLine("All tests passed.");
   }
 }