Resolve "Work on const / non-const in wolf base classes"

Closes #184 (closed)

include/core/trajectory/trajectory_base.h

@@ -170,21 +170,29 @@ inline const FramePtrMap& TrajectoryBase::getFrameMap()
 
 inline FrameBaseConstPtr TrajectoryBase::getFirstFrame() const
 {
+    if (frame_const_map_.empty())
+        return nullptr;
     return frame_const_map_.begin()->second;
 }
 
 inline FrameBasePtr TrajectoryBase::getFirstFrame()
 {
+    if (frame_map_.empty())
+        return nullptr;
     return frame_map_.begin()->second;
 }
 
 inline FrameBaseConstPtr TrajectoryBase::getLastFrame() const
 {
+    if (frame_const_map_.empty())
+        return nullptr;
     return frame_const_map_.rbegin()->second;
 }
 
 inline FrameBasePtr TrajectoryBase::getLastFrame()
 {
+    if (frame_map_.empty())
+        return nullptr;
     return frame_map_.rbegin()->second;
 }