Fixed code smell + added test

Qwirkle/QwirkleClassLibrary/Boards/Cell.cs

@@ -61,7 +61,7 @@ public class Cell : INotifyPropertyChanged
     /// <returns>True if the cell is empty, false if the cell contains a tile.</returns>
     public bool IsFree
     {
-        get { return Tile == null; }
+        get { return Tile! == null!; }
     }
 
 
@@ -72,7 +72,7 @@ public class Cell : INotifyPropertyChanged
     /// <returns>True if added succefully (if the cell didn't already contain a tile), false if there already was a tile in this cell.</returns>
     public bool SetTile(Tile addedTile)
     {
-        if (Tile == null)
+        if (Tile! == null!)
         {
 
             Tile = addedTile;

Qwirkle/QwirkleClassLibrary/Games/Game.cs

@@ -559,7 +559,7 @@ namespace QwirkleClassLibrary.Games
         /// <param name="nbTiles"></param>
         /// <param name="checkdoubles"></param>
         /// <returns></returns>
-        private static bool CheckTileInCompletedLines(Tile? t1, ref int nbTiles, ref List<Tile> checkdoubles)
+        public static bool CheckTileInCompletedLines(Tile? t1, ref int nbTiles, ref List<Tile> checkdoubles)
         {
             if (t1! != null!)
             {

Qwirkle/QwirkleClassLibrary/Tiles/Tile.cs

@@ -62,7 +62,7 @@ namespace QwirkleClassLibrary.Tiles
         {
             if (obj == null)
             {
-                throw new NullReferenceException();
+                throw new NullReferenceException("The object is null.");
             }
             
             var otherTile = obj as Tile;

Qwirkle/TestBase/TestGame.cs

@@ -381,6 +381,23 @@ public class TestGame
 
     }
 
+    [Fact]
+    public void Test_CheckTileInCompletedLines()
+    {
+        
+        int nbTiles = 0;
+        var checkdoubles = new List<Tile>()
+        {
+            new(Shape.Club, Color.Blue),
+            new(Shape.Club, Color.Red),
+            new(Shape.Club, Color.Green),
+        };
+        
+        var t1 = new Tile(Shape.Club, Color.Green);
+        
+        Assert.False(Game.CheckTileInCompletedLines(t1, ref nbTiles, ref checkdoubles));
+    }
+
     [Theory]
     [InlineData(3, 1, 4, 1, 5, 1, 5)]
     [InlineData(2, 2, 3, 2, 4, 2, 5)]