Fix the completion when base type is unknown type from unknown namespace

Fixes microsoft#25387

Author: sheetalkamat

src/services/completions.ts

@@ -1653,8 +1653,8 @@ namespace ts.Completions {
             if (!(classElementModifierFlags & ModifierFlags.Private)) {
                 // List of property symbols of base type that are not private and already implemented
                 const baseSymbols = flatMap(getAllSuperTypeNodes(decl), baseTypeNode => {
-                    const type = typeChecker.getTypeAtLocation(baseTypeNode)!; // TODO: GH#18217
-                    return typeChecker.getPropertiesOfType(classElementModifierFlags & ModifierFlags.Static ? typeChecker.getTypeOfSymbolAtLocation(type.symbol, decl) : type);
+                    const type = typeChecker.getTypeAtLocation(baseTypeNode);
+                    return type && typeChecker.getPropertiesOfType(classElementModifierFlags & ModifierFlags.Static ? typeChecker.getTypeOfSymbolAtLocation(type.symbol, decl) : type);
                 });
                 symbols = filterClassMembersList(baseSymbols, decl.members, classElementModifierFlags);
             }

tests/cases/fourslash/completionListClassMembersWithSuperClassFromUnknownNamespace.ts

@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+////class Child extends Namespace.Parent {
+////    /**/
+////}
+
+goTo.marker("");
+verify.completionListContainsClassElementKeywords();
+verify.completionListCount(verify.allowedClassElementKeywords.length);