When executing the following code, it results in a NullPointerException. 
It is supposed to copy the content of cells from another workbook sheet into `workbook`. Calling `setCellFormula(String)` without the external reference does work. 

```scala
import org.apache.poi.ss.usermodel.CellType
import org.apache.poi.xssf.usermodel.XSSFWorkbook

object Main {
  def main(args: Array[String]): Unit = {

    val workbook = new XSSFWorkbook()
    val evaluator = workbook.getCreationHelper.createFormulaEvaluator()
    evaluator.setIgnoreMissingWorkbooks(true)

    val sheet = workbook.createSheet("sheet")
    val row = sheet.createRow(0)
    val cell = row.createCell(0, CellType.FORMULA)

    try {
          cell.setCellFormula(f"[some-workbook-that-does-not-yet-exist.xlsx]main!B:D")
    } catch {
      case e: Throwable => e.printStackTrace()
    }
  }
}
```

Here is the Stacktrace :

```
java.lang.NullPointerException
        at org.apache.poi.xssf.usermodel.BaseXSSFEvaluationWorkbook.findExternalLinkIndex(BaseXSSFEvaluationWorkbook.java:140)
        at org.apache.poi.xssf.usermodel.BaseXSSFEvaluationWorkbook.resolveBookIndex(BaseXSSFEvaluationWorkbook.java:114)
        at org.apache.poi.xssf.usermodel.BaseXSSFEvaluationWorkbook.get3DReferencePtg(BaseXSSFEvaluationWorkbook.java:268)
        at org.apache.poi.ss.formula.FormulaParser.createAreaRefParseNode(FormulaParser.java:992)
        at org.apache.poi.ss.formula.FormulaParser.parseRangeable(FormulaParser.java:528)
        at org.apache.poi.ss.formula.FormulaParser.parseRangeExpression(FormulaParser.java:322)
        at org.apache.poi.ss.formula.FormulaParser.parseSimpleFactor(FormulaParser.java:1548)
        at org.apache.poi.ss.formula.FormulaParser.percentFactor(FormulaParser.java:1506)
        at org.apache.poi.ss.formula.FormulaParser.powerFactor(FormulaParser.java:1493)
        at org.apache.poi.ss.formula.FormulaParser.Term(FormulaParser.java:1867)
        at org.apache.poi.ss.formula.FormulaParser.additiveExpression(FormulaParser.java:1994)
        at org.apache.poi.ss.formula.FormulaParser.concatExpression(FormulaParser.java:1978)
        at org.apache.poi.ss.formula.FormulaParser.comparisonExpression(FormulaParser.java:1935)
        at org.apache.poi.ss.formula.FormulaParser.intersectionExpression(FormulaParser.java:1908)
        at org.apache.poi.ss.formula.FormulaParser.unionExpression(FormulaParser.java:1889)
        at org.apache.poi.ss.formula.FormulaParser.parse(FormulaParser.java:2036)
        at org.apache.poi.ss.formula.FormulaParser.parse(FormulaParser.java:170)
        at org.apache.poi.xssf.usermodel.XSSFCell.setFormula(XSSFCell.java:553)
        at org.apache.poi.xssf.usermodel.XSSFCell.setCellFormulaImpl(XSSFCell.java:529)
        at org.apache.poi.ss.usermodel.CellBase.setCellFormula(CellBase.java:126)
        at Main$.main(Main.scala:25)
        at Main.main(Main.scala)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at sbt.Run.invokeMain(Run.scala:133)
        at sbt.Run.execute$1(Run.scala:82)
        at sbt.Run.$anonfun$runWithLoader$5(Run.scala:110)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
        at sbt.util.InterfaceUtil$$anon$1.get(InterfaceUtil.scala:17)
        at sbt.TrapExit$App.run(TrapExit.scala:258)
        at java.base/java.lang.Thread.run(Thread.java:834)
```