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 userCompNode.createError(
                new IllegalArgumentException(
                    "invalid assignment: cannot assign a value to " + operation.name + " operation " + "[" + operation.symbol + "]"
                )
            );
        }
        if (semanticScope.getCondition(userCompNode, Read.class) == false) {
            throw userCompNode.createError(
                new IllegalArgumentException(
                    "not a statement: result not used from " + operation.name + " operation " + "[" + operation.symbol + "]"
                )
            );
        }
        AExpression userLeftNode = userCompNode.getLeftNode();
        semanticScope.setCondition(userLeftNode, Read.class);
        checkedVisit(userLeftNode, semanticScope);
        Class<?> leftValueType = semanticScope.getDecoration(userLeftNode, ValueType.class).valueType();
        AExpression userRightNode = userCompNode.getRightNode();
        semanticScope.setCondition(userRightNode, Read.class);
        checkedVisit(userRightNode, semanticScope);
        Class<?> rightValueType = semanticScope.getDecoration(userRightNode, ValueType.class).valueType();
        Class<?> promotedType;
        if (operation == Operation.EQ || operation == Operation.EQR || operation == Operation.NE || operation == Operation.NER) {
            promotedType = AnalyzerCaster.promoteEquality(leftValueType, rightValueType);
        } else if (operation == Operation.GT || operation == Operation.GTE || operation == Operation.LT || operation == Operation.LTE) {
            promotedType = AnalyzerCaster.promoteNumeric(leftValueType, rightValueType, true);
        } else {
            throw userCompNode.createError(new IllegalStateException("unexpected binary operation [" + operation.name + "]"));
        }
        if (promotedType == null) {