Clang: Fix accessing nullptr

Change-Id: I204ff9263520dee70a89adf99182d198e4dd1036
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/clangcodemodel/clangoverviewmodel.cpp

@@ -81,6 +81,7 @@ void buildTree(const TokenContainers &containers,
         if (parentType == ClangBackEnd::HighlightingType::VirtualFunction
                 || parentType == ClangBackEnd::HighlightingType::Function) {
             // Treat everything inside a function scope as local variables.
+            treeItemCache.erase(index);
             continue;
         }