[Crash] "java.lang.NullPointerException: Ticking memory connection" on loading a dimension with debug chunk generator

The bug

When loading a user-defined dimension generated by debug chunk generator, the game crashes.

How to reproduce

1. Create a new world with the following settings:

   {
     "seed": 1,
     "generate_features": false,
     "dimensions": {
       "minecraft:overworld": {
         "generator": {
           "type": "minecraft:debug"
         },
         "type": {
           "fixed_time": 0,
           "has_skylight": false,
           "has_ceiling": false,
           "ultrawarm": false,
           "natural": false,
           "shrunk": false,
           "ambient_light": 0.0
         }
       },
       "mc-184757:debug": {
         "generator": {
           "type": "minecraft:debug"
         },
         "type": {
           "fixed_time": 0,
           "has_skylight": false,
           "has_ceiling": false,
           "ultrawarm": false,
           "natural": false,
           "shrunk": false,
           "ambient_light": 0.0
         }
       }
     }
   }

   {
     "seed": 1,
     "generate_features": false,
     "dimensions": {
       "minecraft:overworld": {
         "generator": {
           "type": "minecraft:debug"
         },
         "type": {
           "fixed_time": 0,
           "has_skylight": false,
           "has_ceiling": false,
           "ultrawarm": false,
           "natural": false,
           "shrunk": false,
           "ambient_light": 0.0
         }
       },
       "mc-184757:debug": {
         "generator": {
           "type": "minecraft:debug"
         },
         "type": {
           "fixed_time": 0,
           "has_skylight": false,
           "has_ceiling": false,
           "ultrawarm": false,
           "natural": false,
           "shrunk": false,
           "ambient_light": 0.0
         }
       }
     }
   }

2. Move to the dimension mc-184757:debug:

   /execute in mc-184757:debug run teleport 0 0 0

   /execute in mc-184757:debug run teleport 0 0 0

3. Close the world

4. Open the world
   → ❌ The game crashes

Crash report

20w21a\: [^crash-2020-05-21_02.35.16-server.txt]

Description: Ticking memory connection

java.lang.NullPointerException: Ticking memory connection
	at abh.a(SourceFile:152)
	at zq.c(SourceFile:111)
	at zq.b(SourceFile:63)
	at me.a(SourceFile:230)
	at zn.c(SourceFile:171)
	at net.minecraft.server.MinecraftServer.b(SourceFile:905)
	at net.minecraft.server.MinecraftServer.a(SourceFile:832)
	at enr.a(SourceFile:89)
	at net.minecraft.server.MinecraftServer.run(SourceFile:691)
	at java.base/java.lang.Thread.run(Thread.java:832)

Description: Ticking memory connection

java.lang.NullPointerException: Ticking memory connection
	at abh.a(SourceFile:152)
	at zq.c(SourceFile:111)
	at zq.b(SourceFile:63)
	at me.a(SourceFile:230)
	at zn.c(SourceFile:171)
	at net.minecraft.server.MinecraftServer.b(SourceFile:905)
	at net.minecraft.server.MinecraftServer.a(SourceFile:832)
	at enr.a(SourceFile:89)
	at net.minecraft.server.MinecraftServer.run(SourceFile:691)
	at java.base/java.lang.Thread.run(Thread.java:832)