gopls/internal/mcp: fix panic in symbolReferencesHandler on nil types.Object

resolveSymbol  might be nil for unresolved names or members, which
caused ObjectLocation to panic. Added proper nil checks and returned
an error instead of nil to prevent crashes.

Fixes golang/go#74904

Change-Id: I85e5584a2fb8f61baf03e9057089ab8818d07ab4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/697315
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ANAS727189

gopls/internal/mcp/symbol_references.go

@@ -140,15 +140,25 @@ func resolveSymbol(path []string, pkg *cache.Package, pgf *parsego.File) (types.
 	switch len(path) {
 	case 1:
 		_, target := fileScope.LookupParent(path[0], token.NoPos)
+		if target == nil {
+			return nil, fmt.Errorf("failed to resolve name %q", path[0])
+		}
 		return target, nil
 	case 2:
 		switch _, obj := fileScope.LookupParent(path[0], token.NoPos); obj := obj.(type) {
 		case *types.PkgName:
-			return obj.Imported().Scope().Lookup(path[1]), nil
+			target := obj.Imported().Scope().Lookup(path[1])
+			if target == nil {
+				return nil, fmt.Errorf("failed to resolve member %q of %q", path[1], path[0])
+			}
+			return target, nil
 		case nil:
 			return nil, fmt.Errorf("failed to resolve name %q", path[0])
 		default:
 			target, _, _ := types.LookupFieldOrMethod(obj.Type(), true, pkg.Types(), path[1])
+			if target == nil {
+				return nil, fmt.Errorf("failed to resolve member %q of %q", path[1], path[0])
+			}
 			return target, nil
 		}
 	case 3:
@@ -163,6 +173,9 @@ func resolveSymbol(path []string, pkg *cache.Package, pgf *parsego.File) (types.
 			return nil, fmt.Errorf("invalid qualified symbol: could not find %q in package %q", path[1], path[0])
 		}
 		target, _, _ := types.LookupFieldOrMethod(recv.Type(), true, pkg.Types(), path[2])
+		if target == nil {
+			return nil, fmt.Errorf("failed to resolve member %q of %q", path[2], path[1])
+		}
 		return target, nil
 	}
 	panic("unreachable")