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
throw userUnaryNode.createError(
                new IllegalArgumentException(
                    "not a statement: result not used from " + operation.name + " operation " + "[" + operation.symbol + "]"
                )
            );
        }
        AExpression userChildNode = userUnaryNode.getChildNode();
        Class<?> valueType;
        Class<?> unaryType = null;
        if (operation == Operation.SUB && (userChildNode instanceof ENumeric || userChildNode instanceof EDecimal)) {
            semanticScope.setCondition(userChildNode, Read.class);
            semanticScope.copyDecoration(userUnaryNode, userChildNode, TargetType.class);
            semanticScope.replicateCondition(userUnaryNode, userChildNode, Explicit.class);
            semanticScope.replicateCondition(userUnaryNode, userChildNode, Internal.class);
            semanticScope.setCondition(userChildNode, Negate.class);
            checkedVisit(userChildNode, semanticScope);
            if (semanticScope.hasDecoration(userUnaryNode, TargetType.class)) {
                decorateWithCast(userChildNode, semanticScope);
            }
            valueType = semanticScope.getDecoration(userChildNode, ValueType.class).valueType();
        } else {
            if (operation == Operation.NOT) {
                semanticScope.setCondition(userChildNode, Read.class);
                semanticScope.putDecoration(userChildNode, new TargetType(boolean.class));
                checkedVisit(userChildNode, semanticScope);
                decorateWithCast(userChildNode, semanticScope);
                valueType = boolean.class;
            } else if (operation == Operation.BWNOT || operation == Operation.ADD || operation == Operation.SUB) {
                semanticScope.setCondition(userChildNode, Read.class);
                checkedVisit(userChildNode, semanticScope);