Fix Grid to support None/null texture and fix error message bug

- Allow Grid to be created with None as texture parameter
- Use default cell dimensions (16x16) when no texture provided
- Skip sprite rendering when texture is null, but still render colors
- Fix issue #77: Corrected copy/paste error in Grid.at() error messages
- Grid now functional for color-only rendering and entity positioning

Test created to verify Grid works without texture, showing colored cells.

Closes #77

tests/ui_Grid_none_texture_test.py

@@ -0,0 +1,97 @@
+#!/usr/bin/env python3
+"""Test Grid creation with None texture - should work with color cells only"""
+import mcrfpy
+from mcrfpy import automation
+import sys
+
+def test_grid_none_texture(runtime):
+    """Test Grid functionality without texture"""
+    print("\n=== Testing Grid with None texture ===")
+    
+    # Test 1: Create Grid with None texture
+    try:
+        grid = mcrfpy.Grid(10, 10, None, mcrfpy.Vector(50, 50), mcrfpy.Vector(400, 400))
+        print("✓ Grid created successfully with None texture")
+    except Exception as e:
+        print(f"✗ Failed to create Grid with None texture: {e}")
+        sys.exit(1)
+    
+    # Add to UI
+    ui = mcrfpy.sceneUI("grid_none_test")
+    ui.append(grid)
+    
+    # Test 2: Verify grid properties
+    try:
+        grid_size = grid.grid_size
+        print(f"✓ Grid size: {grid_size}")
+        
+        # Check texture property
+        texture = grid.texture
+        if texture is None:
+            print("✓ Grid texture is None as expected")
+        else:
+            print(f"✗ Grid texture should be None, got: {texture}")
+    except Exception as e:
+        print(f"✗ Property access failed: {e}")
+    
+    # Test 3: Access grid points and set colors
+    try:
+        # Create a checkerboard pattern with colors
+        for x in range(10):
+            for y in range(10):
+                point = grid.at(x, y)
+                if (x + y) % 2 == 0:
+                    point.color = mcrfpy.Color(255, 0, 0, 255)  # Red
+                else:
+                    point.color = mcrfpy.Color(0, 0, 255, 255)  # Blue
+        print("✓ Successfully set grid point colors")
+    except Exception as e:
+        print(f"✗ Failed to set grid colors: {e}")
+    
+    # Test 4: Add entities to the grid
+    try:
+        # Create an entity with its own texture
+        entity_texture = mcrfpy.Texture("assets/kenney_tinydungeon.png", 16, 16)
+        entity = mcrfpy.Entity(mcrfpy.Vector(5, 5), entity_texture, 1, grid)
+        grid.entities.append(entity)
+        print(f"✓ Added entity to grid, total entities: {len(grid.entities)}")
+    except Exception as e:
+        print(f"✗ Failed to add entity: {e}")
+    
+    # Test 5: Test grid interaction properties
+    try:
+        # Test zoom
+        grid.zoom = 2.0
+        print(f"✓ Set zoom to: {grid.zoom}")
+        
+        # Test center
+        grid.center = mcrfpy.Vector(5, 5)
+        print(f"✓ Set center to: {grid.center}")
+    except Exception as e:
+        print(f"✗ Grid properties failed: {e}")
+    
+    # Take screenshot
+    filename = f"grid_none_texture_test_{int(runtime)}.png"
+    result = automation.screenshot(filename)
+    print(f"\nScreenshot saved: {filename} - Result: {result}")
+    print("The grid should show a red/blue checkerboard pattern")
+    
+    print("\n✓ PASS - Grid works correctly without texture!")
+    sys.exit(0)
+
+# Set up test scene
+print("Creating test scene...")
+mcrfpy.createScene("grid_none_test")
+mcrfpy.setScene("grid_none_test")
+
+# Add a background frame so we can see the grid
+ui = mcrfpy.sceneUI("grid_none_test")
+background = mcrfpy.Frame(0, 0, 800, 600,
+                         fill_color=mcrfpy.Color(200, 200, 200),
+                         outline_color=mcrfpy.Color(0, 0, 0),
+                         outline=2.0)
+ui.append(background)
+
+# Schedule test
+mcrfpy.setTimer("test", test_grid_none_texture, 100)
+print("Test scheduled...")