VS Code extension: triggering intellisense in a union field throws an error

**Intended outcome:**

When trying to autocomplete fields in a union type, the extension offers to define an inline fragment (preferred) or otherwise notify me that there are no interesting fields to autocomplete (dispreferred, but acceptable).

**Actual outcome:**

When I try to autocomplete the fields in a union type, the extension throws and pops up the error pane. The autocomplete includes no context-aware options.


An instance of the error:

```
[Error - 11:45:49 AM] Request textDocument/completion failed.
Message: Request textDocument/completion failed with message: parentType.getFields is not a function
Code: -32603
```

**How to reproduce the issue:**

`BindingTargetSpec` is the type of the field that I was trying to autocomplete:

```graphql
type MeasurementBindingTargetSpec {
nodeLocation: String!
nodeType: String
nodeSubtype: String
measurementType: String!
}

type PropertyBindingTargetSpec {
nodeLocation: String!
nodeType: String
nodeSubtype: String
propertyName: String!
}

union BindingTargetSpec = MeasurementBindingTargetSpec | PropertyBindingTargetSpec
```

**Versions**

Extension version: v1.15.3

Start by reproducing the failure through the VS Code extension's textDocument/completion request using the provided BindingTargetSpec union. Compare the intended inline-fragment or no-options behavior with the reported parentType.getFields error. Done means union-field completion no longer throws and provides the expected context-aware result.

Written by the indexing model from the issue text.