Improper conversion from generatorOptions causes Nukkit to crash

[joserobjr]

Affected Version
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2.0.0-SNAPSHOT

Describe the bug
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Nukkit fails to start when using a world which was converted successfully

To Reproduce
Steps to reproduce the behavior:

1. Have a 1.8.9 spigot world
2. Optimize it on 1.14.2
3. Convert to Nukkit without errors
4. Replace the Nukkit world with the converted world
5. Launch Nukkit
6. See error

Expected behavior
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Start the server without issues.

Exception stack trace
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01:20:07 [INFO ] Preparando mundo "world"
> java.lang.ClassCastException: cn.nukkit.nbt.tag.CompoundTag cannot be cast to cn.nukkit.nbt.tag.StringTag
        at cn.nukkit.nbt.tag.CompoundTag.getString(CompoundTag.java:169)
        at cn.nukkit.level.format.generic.BaseLevelProvider$1.<init>(BaseLevelProvider.java:100)
        at cn.nukkit.level.format.generic.BaseLevelProvider.getGeneratorOptions(BaseLevelProvider.java:98)
        at cn.nukkit.level.Level$2.init(Level.java:232)
        at cn.nukkit.level.Level$2.init(Level.java:228)
        at cn.nukkit.utils.IterableThreadLocal.initialValue(IterableThreadLocal.java:21)
        at java.lang.ThreadLocal.setInitialValue(Unknown Source)
        at java.lang.ThreadLocal.get(Unknown Source)
        at cn.nukkit.level.Level.initLevel(Level.java:407)
        at cn.nukkit.Server.loadLevel(Server.java:1885)
        at cn.nukkit.Server.<init>(Server.java:529)
        at cn.nukkit.Nukkit.main(Nukkit.java:112)
01:20:07 [ERROR] Throwing
java.lang.NullPointerException: null
        at cn.nukkit.level.Level.initLevel(Level.java:408) ~[nukkit-1.0-SNAPSHOT.jar:?]
        at cn.nukkit.Server.loadLevel(Server.java:1885) ~[nukkit-1.0-SNAPSHOT.jar:?]
        at cn.nukkit.Server.<init>(Server.java:529) ~[nukkit-1.0-SNAPSHOT.jar:?]
        at cn.nukkit.Nukkit.main(Nukkit.java:112) [nukkit-1.0-SNAPSHOT.jar:?]
01:20:07 [INFO ] Stopping other threads

Screenshots
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Additional context
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[joserobjr]

The issue is caused because the way Java Edition saves the generatorOptions NBT differs from Nukkit and needs a conversion