Prevent LSP server from crashing on code actions (#5543)

Fixes #5541

### Description
- Prevent LSP server from crashing on code actions

### How has this been tested?
- Added an LSP test

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/DafnyLanguageServer.Test/CodeActions/CodeActionsTest.cs

@@ -27,6 +27,19 @@ public class CodeActionTest : ClientBasedLanguageServerTest {
  return await client.ResolveCodeAction(codeAction, CancellationToken);
  }
 
+ [Fact]
+ public async Task MethodKeywordCodeAction() {
+ await SetUp(o => o.Set(CommonOptionBag.RelaxDefiniteAssignment, true));
+
+ MarkupTestFile.GetPositionsAndAnnotatedRanges(@"
+method><".TrimStart(), out var source, out var positions,
+ out var ranges);
+ var documentItem = await CreateOpenAndWaitForResolve(source);
+ var position = positions[0];
+ var completionList = await RequestCodeActionAsync(documentItem, new Range(position, position));
+ Assert.Empty(completionList);
+ }
+
  [Fact]
  public async Task GitIssue4401CorrectInsertionPlace() {
  await TestCodeAction(@"

Source/DafnyLanguageServer/Plugins/DafnyCodeActionProvider.cs

@@ -89,6 +89,12 @@ public abstract class DiagnosticDafnyCodeActionProvider : DafnyCodeActionProvide
  public RangeToken? FindTokenRangeFromLspRange(IDafnyCodeActionInput input, Range range) {
  var start = range.Start;
  var startNode = input.Program.FindNode<Node>(input.Uri.ToUri(), start.ToDafnyPosition());
+ if (startNode == null) {
+ // A program should have FileModuleDefinition nodes whose ranges span the entire contents of files,
+ // But currently those nodes are missing
+ return null;
+ }
+
  var startToken = startNode.CoveredTokens.FirstOrDefault(t => t.line - 1 == start.Line && t.col - 1 == start.Character);
  if (startToken == null) {
  logger.LogError($"Could not find starting token for position {start} in node {startNode}");