[Android Viewer] Fix crash after save dialog

Fixed the crash after save dialog caused by
a possible null pointer related to presentation
mode.

Change-Id: I2fcf254f4382eaa5b565e25679486a5158d56eac
Reviewed-on: https://gerrit.libreoffice.org/39847
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

android/source/src/java/org/libreoffice/LibreOfficeMainActivity.java

@@ -414,7 +414,7 @@ public class LibreOfficeMainActivity extends AppCompatActivity implements Settin
                 // noinspection ResultOfMethodCallIgnored
                 mTempFile.delete();
             }
-            if (mTempSlideShowFile.exists()) {
+            if (mTempSlideShowFile != null && mTempSlideShowFile.exists()) {
                 // noinspection ResultOfMethodCallIgnored
                 mTempSlideShowFile.delete();
             }