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
Operation operation = userAssignmentNode.getOperation();
        if (operation != null) {
            checkedVisit(userRightNode, semanticScope);
            Class<?> rightValueType = semanticScope.getDecoration(userRightNode, ValueType.class).valueType();
            Class<?> compoundType;
            boolean isConcatenation = false;
            Class<?> shiftType = null;
            boolean isShift = false;
            if (operation == Operation.MUL) {
                compoundType = AnalyzerCaster.promoteNumeric(leftValueType, rightValueType, true);
            } else if (operation == Operation.DIV) {
                compoundType = AnalyzerCaster.promoteNumeric(leftValueType, rightValueType, true);
            } else if (operation == Operation.REM) {
                compoundType = AnalyzerCaster.promoteNumeric(leftValueType, rightValueType, true);
            } else if (operation == Operation.ADD) {
                compoundType = AnalyzerCaster.promoteAdd(leftValueType, rightValueType);
                isConcatenation = compoundType == String.class;
            } else if (operation == Operation.SUB) {
                compoundType = AnalyzerCaster.promoteNumeric(leftValueType, rightValueType, true);
            } else if (operation == Operation.LSH) {
                compoundType = AnalyzerCaster.promoteNumeric(leftValueType, false);
                shiftType = AnalyzerCaster.promoteNumeric(rightValueType, false);
                isShift = true;
            } else if (operation == Operation.RSH) {
                compoundType = AnalyzerCaster.promoteNumeric(leftValueType, false);
                shiftType = AnalyzerCaster.promoteNumeric(rightValueType, false);
                isShift = true;
            } else if (operation == Operation.USH) {
                compoundType = AnalyzerCaster.promoteNumeric(leftValueType, false);
                shiftType = AnalyzerCaster.promoteNumeric(rightValueType, false);