DonationsActivity - Solution

public class DonationsActivity extends FragmentActivity {
    private DonationsFragment donationsFragment = null;
. . .
// (INSTANTITATE THE DONATIONS FRAGMENT AS IN THE EXAMPLE)
    @Override
    public void onCreate(Bundle savedInstanceState) {
. . .
            this.donationsFragment = DonationsFragment.newInstance(. . .);
. . .
    }
// How I solved it
    @Override
    protected void onPostCreate(@Nullable Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        try {
            Button googleDonationsButton = donationsFragment.getActivity()
                    .findViewById(R.id.donations__google_android_market_donate_button);
            googleDonationsButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    try {
                        donationsFragment.donateGoogleOnClick(v);
                    } catch (IllegalStateException e) {
                        new MaterialDialog.Builder(DonationsActivity.this)
                                .title(R.string.donations__google_android_market_not_supported_title)
                                .content(R.string.donations__google_android_market_not_supported)
                                .cancelable(true)
                                .positiveText(R.string.accept)
                                .build().show();
                    }
                }
            });
        } catch (NullPointerException e) {
            Log.e("DonationsActivity", "Unable to get button-fragment. Full trace: "
            + e.getMessage());
        }
    }