Compile failed, entitycustom cannot be converted to entitycreature

I'm trying to make a slime boss, but I keep getting compile errors. The main issue appears to be that "entitycustom cannot be converted to entitycreature", I have some idea of what to do through browsing the forum, but I'm stuck now so I'm making this. Here's the console message:

Executing gradle command: build
Build info: MCreator 1.9.0, 64-bit, 7638 MB, Windows 10 - cmd, JVM 1.8.0_212, JAVA_HOME: C:\Program Files\Pylo\MCreator\jdk
C:\Users\[redacted]\MCreatorWorkspaces\new_game__mod>gradlew -Dorg.gradle.java.home="C:/Program Files/Pylo/MCreator/jdk" -Dorg.gradle.parallel=false -Dorg.gradle.jvmargs="-Xms625m -Xmx2048m" build 
This mapping 'snapshot_20171003' was designed for MC 1.12! Use at your own peril.
:deobfCompileDummyTask
:deobfProvidedDummyTask
:sourceApiJava UP-TO-DATE
:compileApiJava NO-SOURCE
:processApiResources NO-SOURCE
:apiClasses UP-TO-DATE
:sourceMainJava
C:\Users\[redacted]\MCreatorWorkspaces\new_game__mod\build\sources\main\java\net\mcreator\new_game__mod\MCreatorRedcoreSlime.java:97: error: incompatible types: EntityCustom cannot be converted to EntityCreature
         this.tasks.addTask(1, new EntityAIWander(this, 1));
                                                  ^
C:\Users\[redacted]\MCreatorWorkspaces\new_game__mod\build\sources\main\java\net\mcreator\new_game__mod\MCreatorRedcoreSlime.java:101: error: incompatible types: EntityCustom cannot be converted to EntityCreature
         this.tasks.addTask(5, new EntityAIPanic(this, 1.2));
                                                 ^
C:\Users\[redacted]\MCreatorWorkspaces\new_game__mod\build\sources\main\java\net\mcreator\new_game__mod\MCreatorRedcoreSlime.java:102: error: incompatible types: EntityCustom cannot be converted to EntityCreature
         this.targetTasks.addTask(6, new EntityAIHurtByTarget(this, false));
                                                              ^
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
3 errors
:compileJava FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':compileJava'.
> Compilation failed; see the compiler error output for details.
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
* Get more help at https://help.gradle.org
5 actionable tasks: 4 executed, 1 up-to-date
BUILD FAILED in 13s
C:\Users\[redacted]\MCreatorWorkspaces\new_game__mod>
Task completed with return code 0 in 15574 milliseconds