Erro

 --------- beginning of crash
2019-01-18 15:56:03.659 7457-7457/br.com.teste.agenda E/AndroidRuntime: FATAL EXCEPTION: main
    Process: br.com.teste.agenda, PID: 7457
    java.lang.RuntimeException: android.os.NetworkOnMainThreadException
        at br.com.teste.agenda.WebClient.post(WebClient.java:33)
        at br.com.teste.agenda.ListaAlunosActivity.onOptionsItemSelected(ListaAlunosActivity.java:126)
        at android.app.Activity.onMenuItemSelected(Activity.java:3204)
        at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:407)
        at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:195)
        at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:108)
        at android.support.v7.app.AppCompatDelegateImplV9.onMenuItemSelected(AppCompatDelegateImplV9.java:674)
        at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
        at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:963)
        at android.support.v7.widget.ActionMenuView.invokeItem(ActionMenuView.java:624)
        at android.support.v7.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:150)
        at android.view.View.performClick(View.java:5637)
        at android.view.View$PerformClick.run(View.java:22429)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6119)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
     Caused by: android.os.NetworkOnMainThreadException
        at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1303)
        at java.net.Inet6AddressImpl.lookupHostByName(Inet6AddressImpl.java:86)
        at java.net.Inet6AddressImpl.lookupAllHostAddr(Inet6AddressImpl.java:74)
        at java.net.InetAddress.getAllByName(InetAddress.java:752)
        at com.android.okhttp.internal.Network$1.resolveInetAddresses(Network.java:29)
        at com.android.okhttp.internal.http.RouteSelector.resetNextInetSocketAddress(RouteSelector.java:187)
        at com.android.okhttp.internal.http.RouteSelector.nextProxy(RouteSelector.java:156)
        at com.android.okhttp.internal.http.RouteSelector.next(RouteSelector.java:98)
        at com.android.okhttp.internal.http.HttpEngine.createNextConnection(HttpEngine.java:345)
        at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:328)
        at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:246)
        at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:457)
        at com.android.okhttp.internal.huc.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:126)
        at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getOutputStream(HttpURLConnectionImpl.java:257)
        at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.getOutputStream(DelegatingHttpsURLConnection.java:218)
        at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java)
        at br.com.teste.agenda.WebClient.post(WebClient.java:22)
        at br.com.teste.agenda.ListaAlunosActivity.onOptionsItemSelected(ListaAlunosActivity.java:126) 
        at android.app.Activity.onMenuItemSelected(Activity.java:3204) 
        at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:407) 
        at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:195) 
        at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:108) 
        at android.support.v7.app.AppCompatDelegateImplV9.onMenuItemSelected(AppCompatDelegateImplV9.java:674) 
        at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822) 
        at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171) 
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973) 
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:963) 
        at android.support.v7.widget.ActionMenuView.invokeItem(ActionMenuView.java:624) 
        at android.support.v7.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:150) 
        at android.view.View.performClick(View.java:5637) 
        at android.view.View$PerformClick.run(View.java:22429) 
        at android.os.Handler.handleCallback(Handler.java:751) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6119) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
    
    
    --------- beginning of system