Fix crash when filtering moves that are off the board

assets/export/edge.png.import

@@ -0,0 +1,34 @@
+[remap]
+
+importer="texture"
+type="StreamTexture"
+path="res://.import/edge.png-f8eb0bb54aa0cd5dd98a7aca825fc2d4.stex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://assets/export/edge.png"
+dest_files=[ "res://.import/edge.png-f8eb0bb54aa0cd5dd98a7aca825fc2d4.stex" ]
+
+[params]
+
+compress/mode=0
+compress/lossy_quality=0.7
+compress/hdr_mode=0
+compress/bptc_ldr=0
+compress/normal_map=0
+flags/repeat=0
+flags/filter=true
+flags/mipmaps=false
+flags/anisotropic=false
+flags/srgb=2
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/HDR_as_SRGB=false
+process/invert_color=false
+stream=false
+size_limit=0
+detect_3d=true
+svg/scale=1.0

src/Game.gd

@@ -65,7 +65,13 @@ func get_valid_piece_moves(piece):
 	# @TODO Filter based on game state
 	var moves = []
 	for m in possible_moves:
-		var target_square = self.board_squares[m['pos']]
+		var target_square = null
+		if self.board_squares.has(m['pos']):
+			target_square = self.board_squares[m['pos']]
+		if target_square == null:
+			# Move it off the board
+			print("Move to ", m['pos'], " is not valid due to not being on the board")
+			continue
 		if target_square['piece']:
 			# something here
 			if pieces_hostile(piece, target_square['piece']):