Why is it not possible to see my public activity in an external runnable class?

Question

When the bluetooth connection turns on, I want to start a Therad which is checking for the available devices. When starting this Thread, I want to start an other activity. I created a public method with an intend to start the activity and now I want to call it from my Thread, but it isn't visable.

MainActivity Content

 public Thread cConnectBT = new Thread();


    public void vBT_popup_start()
    {
        Handler hBT_popup_start = new Handler(Looper.getMainLooper());
        hBT_popup_start.post(new Runnable() {
            @Override
            public void run() {
                Intent tent_BT_popup_Start = new Intent (MainActivity.this, cBT_popup.class);
                startActivity(tent_BT_popup_Start);
            }
        });

    }
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#Thread Content


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public class cConnectBT implements Runnable {

    @Override
    public void run() {
           vBT_popup_start();
        }

    }


}

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vBT_popup_start();
is not visable, but why? how to fix that?