fix NPE when baseUrl is missing

no idea how this happens, however this was
reported via gplay pre launch report for 20.0.0RC1
("Detected on 10 devices during testing"):

Exception java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.nextcloud.talk.data.user.model.User.getBaseUrl()' on a null object reference
  at com.nextcloud.talk.activities.BaseActivity.startActivity (BaseActivity.kt:240)
  at com.nextcloud.talk.account.ServerSelectionActivity.showVisitProvidersInfo$lambda$5 (ServerSelectionActivity.kt:206)
  at com.nextcloud.talk.account.ServerSelectionActivity.$r8$lambda$pjpPT-LQbGLSCJPXeRE8IJvpLIE
  at com.nextcloud.talk.account.ServerSelectionActivity$$ExternalSyntheticLambda0.onClick (D8$$SyntheticClass)
  at android.view.View.performClick (View.java:7506)
  at android.view.View.performClickInternal (View.java:7483)
  at android.view.View.-$$Nest$mperformClickInternal
  at android.view.View$PerformClick.run (View.java:29335)
  at android.os.Handler.handleCallback (Handler.java:942)
  at android.os.Handler.dispatchMessage (Handler.java:99)
  at androidx.test.espresso.base.Interrogator.loopAndInterrogate (Interrogator.java:10)
  at androidx.test.espresso.base.UiControllerImpl.loopUntil (UiControllerImpl.java:7)
  at androidx.test.espresso.base.UiControllerImpl.loopUntil (UiControllerImpl.java:1)
  at androidx.test.espresso.base.UiControllerImpl.injectMotionEvent (UiControllerImpl.java:5)
  at androidx.test.espresso.action.MotionEvents.sendUp (MotionEvents.java:6)
  at androidx.test.espresso.action.MotionEvents.sendUp (MotionEvents.java:1)
  at androidx.test.espresso.action.Tap.sendSingleTap (Tap.java:5)
  at androidx.test.espresso.action.Tap.-$$Nest$smsendSingleTap
  at androidx.test.espresso.action.Tap$1.sendTap (Tap.java:1)
  at androidx.test.espresso.action.GeneralClickAction.perform (GeneralClickAction.java:4)
  at androidx.test.espresso.ViewInteraction$SingleExecutionViewAction.perform (ViewInteraction.java:2)
  at androidx.test.espresso.ViewInteraction.doPerform (ViewInteraction.java:23)
  at androidx.test.espresso.ViewInteraction.-$$Nest$mdoPerform
  at androidx.test.espresso.ViewInteraction$1.call (ViewInteraction.java:6)
  at androidx.test.espresso.ViewInteraction$1.call (ViewInteraction.java:1)
  at java.util.concurrent.FutureTask.run (FutureTask.java:264)
  at android.os.Handler.handleCallback (Handler.java:942)
  at android.os.Handler.dispatchMessage (Handler.java:99)
  at android.os.Looper.loopOnce (Looper.java:201)
  at android.os.Looper.loop (Looper.java:288)
  at android.app.ActivityThread.main (ActivityThread.java:7898)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:548)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:936)

If baseUrl is really missing this may lead to followup issues, however this maybe only 'happens' in gplay pre launch report without any real world scenario. A best solution may be to make baseUrl not nullable, but don't want to do this on short term before release..

Signed-off-by: Marcel Hibbe <dev@mhibbe.de>

app/src/main/java/com/nextcloud/talk/activities/BaseActivity.kt

@@ -237,7 +237,7 @@ open class BaseActivity : AppCompatActivity() {
         val user = currentUserProvider.currentUser.blockingGet()
         if (intent.data != null && TextUtils.equals(intent.action, Intent.ACTION_VIEW)) {
             val uri = intent.data.toString()
-            if (uri.startsWith(user.baseUrl!!)) {
+            if (user?.baseUrl != null && uri.startsWith(user.baseUrl!!)) {
                 if (UriUtils.isInstanceInternalFileShareUrl(user.baseUrl!!, uri)) {
                     // https://cloud.nextcloud.com/f/41
                     val fileViewerUtils = FileViewerUtils(applicationContext, user)