Fix NullPointerException

app/src/main/java/org/isoron/uhabits/models/CheckmarkList.java

@@ -192,7 +192,7 @@ public abstract class CheckmarkList
         Checkmark newest = getNewestComputed();
         Checkmark oldest = getOldestComputed();
 
-        if (newest == null)
+        if (newest == null || oldest == null)
         {
             forceRecompute(from, to);
         }
@@ -208,6 +208,7 @@ public abstract class CheckmarkList
      *
      * @return oldest checkmark already computed
      */
+    @Nullable
     protected abstract Checkmark getOldestComputed();
 
     /**
@@ -285,5 +286,6 @@ public abstract class CheckmarkList
      *
      * @return newest checkmark already computed
      */
+    @Nullable
     protected abstract Checkmark getNewestComputed();
 }

app/src/main/java/org/isoron/uhabits/models/memory/MemoryCheckmarkList.java

@@ -72,6 +72,7 @@ public class MemoryCheckmarkList extends CheckmarkList
     }
 
     @Override
+    @Nullable
     protected Checkmark getOldestComputed()
     {
         if(list.isEmpty()) return null;
@@ -79,6 +80,7 @@ public class MemoryCheckmarkList extends CheckmarkList
     }
 
     @Override
+    @Nullable
     protected Checkmark getNewestComputed()
     {
         if(list.isEmpty()) return null;

app/src/main/java/org/isoron/uhabits/models/sqlite/SQLiteCheckmarkList.java

@@ -153,6 +153,7 @@ public class SQLiteCheckmarkList extends CheckmarkList
     }
 
     @Override
+    @Nullable
     protected Checkmark getOldestComputed()
     {
         check(habit.getId());