Cherry-pick PR microsoft#47395 into release-4.5

Component commits:
f230a5a Add failing test.

3ba8469 Guard against undefined module bodies in navbar/navtree.

src/services/navigationBar.ts

@@ -650,7 +650,10 @@ namespace ts.NavigationBar {
     // We use 1 NavNode to represent 'A.B.C', but there are multiple source nodes.
     // Only merge module nodes that have the same chain. Don't merge 'A.B.C' with 'A'!
     function areSameModule(a: ModuleDeclaration, b: ModuleDeclaration): boolean {
-        return a.body!.kind === b.body!.kind && (a.body!.kind !== SyntaxKind.ModuleDeclaration || areSameModule(a.body as ModuleDeclaration, b.body as ModuleDeclaration));
+        if (!a.body || !b.body) {
+            return a.body === b.body;
+        }
+        return a.body.kind === b.body.kind && (a.body.kind !== SyntaxKind.ModuleDeclaration || areSameModule(a.body as ModuleDeclaration, b.body as ModuleDeclaration));
     }
 
     /** Merge source into target. Source should be thrown away after this is called. */

tests/cases/fourslash/navbarForDoubleAmbientModules01.ts

@@ -0,0 +1,24 @@
+/// <reference path="./fourslash.ts" />
+
+//// declare module "foo";  
+//// declare module "foo";
+
+verify.navigationBar([
+    {
+        "text": "<global>",
+        "kind": "script",
+        "childItems": [
+            {
+                "text": "\"foo\"",
+                "kind": "module",
+                "kindModifiers": "declare"
+            }
+        ]
+    },
+    {
+        "text": "\"foo\"",
+        "kind": "module",
+        "kindModifiers": "declare",
+        "indent": 1
+    }
+]);