From notebook,how can I change variable value with in .m file dynamically?

Question

I wrote a function named Testing.

BeginPackage["TestPackage`"]
Testing[tabNames_] :=
       (
          DynamicModule[
                        {
                            tabIndex = 0,
                            displayPanel = 0
                        },
                        (
                            {
                            Column[{
                                    (*Setter Bar Part*)
                                    (
                                        Dynamic[
                                                SetterBar[                                                                                                                                                                                                                                                                                                                                     
                                                          Dynamic[tabIndex],
                                                          (Table[(i -> tabNames[[i]]), {i, 1, Length[tabNames]}]),
                                                          {ImageSize -> {110, 50}}
                                                         ]
                                                ]
                                    ),
                                (*Setter bar IS Closed here*)
                                (*middle Panel Part*)
                                Dynamic[
                                            Panel[

                                                displayPanel
                                                ]
                                           ]
                                }
                            ], Dynamic[tabIndex]}(*Final PAnel IS Closed here*)
                        )
                ]
  )
  EndPackage[]

I saved the above code in .m file.

.nb file code

   functionCalling = Testing[{a, b, c, d}];
   notebookList = {1, 2, 3, 4, 5};
   TestPackage`displayPanel =Dynamic[(Part[notebookList, (TestPackage`tabIndex + 1)])];
   Dynamic[Part[functionCalling, 2, 1]]

I want to track the TabIndex value dynamically,If you click on any setterButton.

depending upon the TabIndexvalue,I will find the element from the notebookList and assign to displayPanel.

after assignment,the updated displayPanel value should be displayed dynamically in notebook.

for this purpose,I wrote the above code in a .nb file.But that code is not working

How can I solve this issue?

Please help me?