fix for missing handler

Author: hexaeder

NetworkDynamicsInspector/src/serving.jl

@@ -85,7 +85,7 @@ function serve_app(::BrowserDisp, app)
     nothing
 end
 function close_display(::BrowserDisp; kwargs...)
-    if BROWSER_STATE[:handler] isa Bonito.HTTPServer.BrowserDisplay
+    if haskey(BROWSER_STATE, :handler) && BROWSER_STATE[:handler] isa Bonito.HTTPServer.BrowserDisplay
         close(BROWSER_STATE[:handler])
     end
     BROWSER_STATE[:handler] = nothing