static @Nullable String getFunctionalInterfaceClassName(PsiMethodReferenceExpression expression) {
    PsiType functionalInterfaceType = expression.getFunctionalInterfaceType();
    if (!(functionalInterfaceType instanceof PsiClassType)) return null;
    PsiClass targetType = ((PsiClassType)functionalInterfaceType).resolve();
    if (targetType == null) return null;
    return targetType.getQualifiedName();
  }
static @Nullable String getFunctionalInterfaceClassName(PsiMethodReferenceExpression expression)