NPE in javac 11.0.18 while infering exception type in lambda

[Attila Kelemen]

The following code causes javac to throw an NPE:

```
public class JavacBug {
  interface MyFunction<E extends Throwable> {
    Object apply(Object arg) throws E;
  }

  <E extends Throwable> Object withObj(MyFunction<E> f) throws E {
    return f.apply("X");
  }

  void dummyCall(Object arg) {
  }

  void test() {
    withObj(obj1 -> {
      return withObj(obj2 -> {
        dummyCall(obj1);
        return null;
      });
    });
  }
}
```

I have only tried this with 17 (aside from 11.0.18), but 17 compiles it fine.

The workaround to this issue is to explicitly declare the generic
parameter of the inner `withObj` call.

Attila