Fixed equals

src/main/java/cz/muni/fi/pv168/project/data/category/Category.java

@@ -94,7 +94,7 @@ public class Category implements IdentifiableEntity {
             return false;
         }
         Category category = (Category) o;
-        return Objects.equals(id, category.id);
+        return this.id != null && this.id.equals(category.id);
     }
 
     @Override

src/main/java/cz/muni/fi/pv168/project/data/category/CategoryTime.java

@@ -48,7 +48,7 @@ public class CategoryTime {
             return false;
         }
         CategoryTime that = (CategoryTime) o;
-        return Objects.equals(id, that.id);
+        return this.id != null && this.id.equals(that.id);
     }
 
     public void setHoursSpent(Long hoursSpent) {

src/main/java/cz/muni/fi/pv168/project/data/task/SubTask.java

@@ -61,7 +61,7 @@ public class SubTask implements Completable, IdentifiableEntity {
             return false;
         }
         SubTask subTask = (SubTask) o;
-        return Objects.equals(id, subTask.id);
+        return this.id != null && this.id.equals(subTask.id);
     }
 
     @Override

src/main/java/cz/muni/fi/pv168/project/data/task/Task.java

@@ -219,7 +219,7 @@ public class Task implements Completable, IdentifiableEntity {
             return false;
         }
         Task task = (Task) o;
-        return Objects.equals(id, task.id);
+        return this.id != null && this.id.equals(task.id);
     }
 
     @Override