avoid "no fn clause matching" error in Completion.snippet?

Author: Rogach

apps/language_server/lib/language_server/providers/completion.ex

@@ -1441,7 +1441,7 @@ defmodule ElixirLS.LanguageServer.Providers.Completion do
   end
 
   defp snippet?(item) do
-    item.kind == :snippet || String.match?(item.insert_text, ~r/\${?\d/u)
+    item.kind == :snippet || (item.insert_text != nil and String.match?(item.insert_text, ~r/\${?\d/u))
   end
 
   # As defined by CompletionItemTag in https://microsoft.github.io/language-server-protocol/specifications/specification-current/