Warnings when creating Debug world (Moving pistons)

When creating a new Debug world, the log generates a number of warnings relating to Moving Pistons.

To reproduce, create a fresh Debug world. The errors will appear in the log.

7:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=north,type=normal] but failed at location ej{x=23, y=70, z=153}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=east,type=normal] but failed at location ej{x=23, y=70, z=157}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=north,type=sticky] but failed at location ej{x=23, y=70, z=155}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=east,type=sticky] but failed at location ej{x=23, y=70, z=159}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=down,type=sticky] but failed at location ej{x=23, y=70, z=175}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=south,type=normal] but failed at location ej{x=23, y=70, z=161}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=west,type=normal] but failed at location ej{x=23, y=70, z=165}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=south,type=sticky] but failed at location ej{x=23, y=70, z=163}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=up,type=normal] but failed at location ej{x=23, y=70, z=169}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=west,type=sticky] but failed at location ej{x=23, y=70, z=167}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=down,type=normal] but failed at location ej{x=23, y=70, z=173}
17:59:18	bmx	Server thread	warn	Tried to load a block entity for block Block{minecraft:moving_piston}[facing=up,type=sticky] but failed at location ej{x=23, y=70, z=171}

Note: In previous versions it also logged errors for failing to saving chunks, however it appears this has been fixed.

17:59:18	bnj	Server thread	error	Failed to save chunk
java.lang.NullPointerException
	at he.a(SourceFile:93)
	at bnj.a(SourceFile:347)
	at bnj.a(SourceFile:224)
	at tw.a(SourceFile:96)
	at tw$$Lambda$1610/292907655.accept(Unknown Source)
	at java.lang.Iterable.forEach(Iterable.java:75)
	at tw.a(SourceFile:91)
	at tb.d(SourceFile:282)
	at tc.i_(SourceFile:200)
	at net.minecraft.server.MinecraftServer.w(SourceFile:690)
	at net.minecraft.server.MinecraftServer.v(SourceFile:623)
	at dft.v(SourceFile:157)
	at net.minecraft.server.MinecraftServer.run(SourceFile:526)
	at java.lang.Thread.run(Thread.java:745)