Cannot load Bundle in java assembly jar: object scala.Predef not found

I am trying to create a KSQL UDF serving a Mleap model.
The code for a toy example can be found here: [https://gitlab.com/jan-teichmann/ksql-iris-classifier-udf](https://gitlab.com/jan-teichmann/ksql-iris-classifier-udf )

I am loading a pipeline from a BundleFile following the documentation with
```
val pipeline = (for(bundle <- managed(BundleFile(path))) yield {
bundle.loadMleapBundle().get
}).tried.get.root
```
I have a unit test which confirms that the pipeline works as expected and `sbt test` runs successfully. I then create an uber jar with the sbt assembly plugin and put that in the extension folder of KSQL which then loads and tries to instantiate the UDF. This fails with `scala.ScalaReflectionException: object scala.Predef not found.`

```
ksql-server_1 | Caused by: java.lang.reflect.InvocationTargetException
ksql-server_1 | at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
ksql-server_1 | at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
ksql-server_1 | at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
ksql-server_1 | at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
ksql-server_1 | at ml.combust.mleap.bundle.ops.MleapOp.load(MleapOp.scala:24)
ksql-server_1 | at ml.combust.mleap.bundle.ops.MleapOp.load(MleapOp.scala:16)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer$$anonfun$read$2$$anonfun$apply$3$$anonfun$apply$4.apply(NodeSerializer.scala:106)
ksql-server_1 | at scala.util.Try$.apply(Try.scala:192)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer$$anonfun$read$2$$anonfun$apply$3.apply(NodeSerializer.scala:104)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer$$anonfun$read$2$$anonfun$apply$3.apply(NodeSerializer.scala:102)
ksql-server_1 | at scala.util.Success.flatMap(Try.scala:231)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer$$anonfun$read$2.apply(NodeSerializer.scala:102)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer$$anonfun$read$2.apply(NodeSerializer.scala:101)
ksql-server_1 | at scala.util.Success.flatMap(Try.scala:231)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer.read(NodeSerializer.scala:100)
ksql-server_1 | at ml.combust.bundle.serializer.GraphSerializer$$anonfun$readNode$2.apply(GraphSerializer.scala:57)
ksql-server_1 | at ml.combust.bundle.serializer.GraphSerializer$$anonfun$readNode$2.apply(GraphSerializer.scala:57)
ksql-server_1 | at scala.util.Success.flatMap(Try.scala:231)
ksql-server_1 | at ml.combust.bundle.serializer.GraphSerializer.readNode(GraphSerializer.scala:56)
ksql-server_1 | at ml.combust.bundle.serializer.GraphSerializer$$anonfun$read$1.apply(GraphSerializer.scala:44)
ksql-server_1 | at ml.combust.bundle.serializer.GraphSerializer$$anonfun$read$1.apply(GraphSerializer.scala:44)
ksql-server_1 | at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
ksql-server_1 | at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
ksql-server_1 | at scala.collection.immutable.List.foreach(List.scala:381)
ksql-server_1 | at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
ksql-server_1 | at scala.collection.immutable.List.map(List.scala:285)
ksql-server_1 | at ml.combust.bundle.serializer.GraphSerializer.read(GraphSerializer.scala:44)
ksql-server_1 | at ml.combust.mleap.bundle.ops.PipelineOp$$anon$1.load(PipelineOp.scala:28)
ksql-server_1 | at ml.combust.mleap.bundle.ops.PipelineOp$$anon$1.load(PipelineOp.scala:15)
ksql-server_1 | at ml.combust.bundle.serializer.ModelSerializer$$anonfun$readWithModel$2.apply(ModelSerializer.scala:106)
ksql-server_1 | at ml.combust.bundle.serializer.ModelSerializer$$anonfun$readWithModel$2.apply(ModelSerializer.scala:104)
ksql-server_1 | at scala.util.Success$$anonfun$map$1.apply(Try.scala:237)
ksql-server_1 | at scala.util.Try$.apply(Try.scala:192)
ksql-server_1 | at scala.util.Success.map(Try.scala:237)
ksql-server_1 | at ml.combust.bundle.serializer.ModelSerializer.readWithModel(ModelSerializer.scala:103)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer$$anonfun$read$2.apply(NodeSerializer.scala:102)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer$$anonfun$read$2.apply(NodeSerializer.scala:101)
ksql-server_1 | at scala.util.Success.flatMap(Try.scala:231)
ksql-server_1 | at ml.combust.bundle.serializer.NodeSerializer.read(NodeSerializer.scala:100)
ksql-server_1 | at ml.combust.bundle.serializer.BundleSerializer$$anonfun$read$2.apply(BundleSerializer.scala:55)
ksql-server_1 | at ml.combust.bundle.serializer.BundleSerializer$$anonfun$read$2.apply(BundleSerializer.scala:49)
ksql-server_1 | at scala.util.Success.flatMap(Try.scala:231)
ksql-server_1 | at ml.combust.bundle.serializer.BundleSerializer.read(BundleSerializer.scala:49)
ksql-server_1 | at ml.combust.bundle.BundleFile.load(BundleFile.scala:123)
ksql-server_1 | at ml.combust.mleap.runtime.MleapSupport$MleapBundleFileOps.loadMleapBundle(MleapSupport.scala:25)
ksql-server_1 | at ksql.irisudf.Iris$$anonfun$2.apply(IrisClassifier.scala:27)
ksql-server_1 | at ksql.irisudf.Iris$$anonfun$2.apply(IrisClassifier.scala:26)
ksql-server_1 | at resource.AbstractManagedResource$$anonfun$5.apply(AbstractManagedResource.scala:88)
ksql-server_1 | at scala.util.control.Exception$Catch$$anonfun$either$1.apply(Exception.scala:125)
ksql-server_1 | at scala.util.control.Exception$Catch$$anonfun$either$1.apply(Exception.scala:125)
ksql-server_1 | at scala.util.control.Exception$Catch.apply(Exception.scala:103)
ksql-server_1 | at scala.util.control.Exception$Catch.either(Exception.scala:125)
ksql-server_1 | at resource.AbstractManagedResource.acquireFor(AbstractManagedResource.scala:88)
ksql-server_1 | at resource.ManagedResourceOperations$class.apply(ManagedResourceOperations.scala:26)
ksql-server_1 | at resource.AbstractManagedResource.apply(AbstractManagedResource.scala:50)
ksql-server_1 | at resource.DeferredExtractableManagedResource$$anonfun$tried$1.apply(AbstractManagedResource.scala:33)
ksql-server_1 | at scala.util.Try$.apply(Try.scala:192)
ksql-server_1 | at resource.DeferredExtractableManagedResource.tried(AbstractManagedResource.scala:33)
ksql-server_1 | at ksql.irisudf.Iris.(IrisClassifier.scala:29)
ksql-server_1 | at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
ksql-server_1 | at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
ksql-server_1 | at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
ksql-server_1 | at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
ksql-server_1 | at java.lang.Class.newInstance(Class.java:442)
ksql-server_1 | at io.confluent.ksql.function.FunctionLoaderUtils.instantiateFunctionInstance(FunctionLoaderUtils.java:106)
ksql-server_1 | ... 27 more
ksql-server_1 | Caused by: scala.ScalaReflectionException: object scala.Predef not found.
ksql-server_1 | at scala.reflect.internal.Mirrors$RootsBase.staticModule(Mirrors.scala:162)
ksql-server_1 | at scala.reflect.internal.Mirrors$RootsBase.staticModule(Mirrors.scala:22)
ksql-server_1 | at ml.combust.mleap.runtime.transformer.feature.ReverseStringIndexer$$typecreator2$1.apply(ReverseStringIndexer.scala:16)
ksql-server_1 | at scala.reflect.api.TypeTags$WeakTypeTagImpl.tpe$lzycompute(TypeTags.scala:232)
ksql-server_1 | at scala.reflect.api.TypeTags$WeakTypeTagImpl.tpe(TypeTags.scala:232)
ksql-server_1 | at ml.combust.mleap.core.reflection.MleapReflection$class.mirrorType(MleapReflection.scala:75)
ksql-server_1 | at ml.combust.mleap.core.reflection.MleapReflection$class.typeSpec(MleapReflection.scala:18)
ksql-server_1 | at ml.combust.mleap.core.reflection.MleapReflection$.typeSpec(MleapReflection.scala:126)
ksql-server_1 | at ml.combust.mleap.runtime.function.UserDefinedFunction$.function1(UserDefinedFunction.scala:42)
ksql-server_1 | at ml.combust.mleap.runtime.transformer.feature.ReverseStringIndexer.(ReverseStringIndexer.scala:16)
ksql-server_1 | ... 92 more
```

Has anyone an idea what might go wrong here?