Java.lang.nullpointerexception Appears in chat at Singleplayer when putting the /kill command and reload the world while loading command

When i put the /kill command in the chat and reload the world, the error Java.lang.nullpointerexception appers after i reload it in red through the command line, suddendly, another player appears in front of me, and it rapidly disappears,then some blocks got the default texture(the violet one) i was not in multiplayer i swear.
What i expected: Game continue correctly
What actually Happened: A ghost player appeared in front of me and some textures got default and mismatched