fix null pointer exception

closes sk22#539

mastodon/src/main/java/org/joinmastodon/android/model/Instance.java

@@ -11,6 +11,7 @@ import java.net.IDN;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 
 @Parcel
 public class Instance extends BaseModel{
@@ -147,6 +148,22 @@ public class Instance extends BaseModel{
 		return pleroma != null;
 	}
 
+	public boolean hasFeature(Feature feature) {
+		Optional<List<String>> pleromaFeatures = Optional.ofNullable(pleroma)
+				.map(p -> p.metadata)
+				.map(m -> m.features);
+
+		return switch (feature) {
+			case BUBBLE_TIMELINE -> pleromaFeatures
+					.map(f -> f.contains("bubble_timeline"))
+					.orElse(false);
+		};
+	}
+
+	public enum Feature {
+		BUBBLE_TIMELINE
+	}
+
 	@Parcel
 	public static class Rule{
 		public String id;

mastodon/src/main/java/org/joinmastodon/android/model/TimelineDefinition.java

@@ -265,7 +265,7 @@ public class TimelineDefinition {
         @Override
         public boolean wantsDefault(AccountSession session) {
             return session.getInstance()
-                    .map(i -> i.isAkkoma() && i.pleroma.metadata.features.contains("bubble_timeline"))
+                    .map(i -> i.hasFeature(Instance.Feature.BUBBLE_TIMELINE))
                     .orElse(false);
         }
     };