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
semanticScope.setCondition(userReturnNode, MethodEscape.class);
        semanticScope.setCondition(userReturnNode, LoopEscape.class);
        semanticScope.setCondition(userReturnNode, AllEscape.class);
    }
    /**
     * Visits an expression that is also considered a statement.
     * Checks: control flow, type validation
     */
    @Override
    public void visitExpression(SExpression userExpressionNode, SemanticScope semanticScope) {
        Class<?> rtnType = semanticScope.getReturnType();
        boolean isVoid = rtnType == void.class;
        boolean lastSource = semanticScope.getCondition(userExpressionNode, LastSource.class);
        AExpression userStatementNode = userExpressionNode.getStatementNode();
        if (lastSource && isVoid == false) {
            semanticScope.setCondition(userStatementNode, Read.class);
        }
        checkedVisit(userStatementNode, semanticScope);
        Class<?> expressionValueType = semanticScope.getDecoration(userStatementNode, ValueType.class).valueType();
        boolean rtn = lastSource && isVoid == false && expressionValueType != void.class;
        if (rtn) {
            semanticScope.putDecoration(userStatementNode, new TargetType(rtnType));
            semanticScope.setCondition(userStatementNode, Internal.class);
            decorateWithCast(userStatementNode, semanticScope);
            semanticScope.setCondition(userExpressionNode, MethodEscape.class);
            semanticScope.setCondition(userExpressionNode, LoopEscape.class);
            semanticScope.setCondition(userExpressionNode, AllEscape.class);
        }
    }
    /**