I got an unexpected internal NPE when playing with this script.

```
POST _scripts/painless/_execute
{
  "script": {
    "source": """
    return /[^\w]+/.split('abc   def higkln').toArray(String[]::new)
    """
  }
}
```
```
{
  "error" : {
    "root_cause" : [
      {
        "type" : "script_exception",
        "reason" : "compile error",
        "script_stack" : [ ],
        "script" : "\n    return /[^\\w]+/.split('abc   def higkln').toArray(String[]::new)\n    ",
        "lang" : "painless"
      }
    ],
    "type" : "script_exception",
    "reason" : "compile error",
    "script_stack" : [ ],
    "script" : "\n    return /[^\\w]+/.split('abc   def higkln').toArray(String[]::new)\n    ",
    "lang" : "painless",
    "caused_by" : {
      "type" : "null_pointer_exception",
      "reason" : "Cannot read field \"annotations\" because the return value of \"org.elasticsearch.painless.lookup.PainlessLookup.lookupPainlessClass(java.lang.Class)\" is null"
    }
  },
  "status" : 400
}

```
modules/lang-painless/src/main/java/org/elasticsearch/painless/phase/DefaultSemanticAnalysisPhase.java
valueType = String.class;
                semanticScope.putDecoration(userFunctionRefNode, EncodingDecoration.of(true, isInstanceReference, symbol, methodName, 0));
            } else {
                FunctionRef ref = FunctionRef.create(
                    scriptScope.getPainlessLookup(),
                    scriptScope.getFunctionTable(),
                    location,
                    targetType.targetType(),
                    symbol,
                    methodName,
                    0,
                    scriptScope.getCompilerSettings().asMap(),
                    isInstanceReference
                );
                valueType = targetType.targetType();
                semanticScope.putDecoration(userFunctionRefNode, new ReferenceDecoration(ref));
            }
        } else {
            if (semanticScope.getCondition(userFunctionRefNode, Write.class)) {
                throw userFunctionRefNode.createError(
                    new IllegalArgumentException(
                        "invalid assignment: cannot assign a value to capturing function reference [" + symbol + ":" + methodName + "]"
                    )
                );
            }
            if (read == false) {
                throw userFunctionRefNode.createError(
                    new IllegalArgumentException(
                        "not a statement: capturing function reference [" + symbol + ":" + methodName + "] not used"
                    )