Change assertions for nullptr if/else checks

Change the assertions so that we can run the visualizer
on a thread and handle incomplete information cases.

src/factor/factor_base.cpp

@@ -151,20 +151,23 @@ void FactorBase::remove(bool viral_remove_empty_parent)
 
 CaptureBasePtr FactorBase::getCapture() const
 {
-    assert(getFeature() != nullptr && "calling getCapture before linking with a feature");
-    return getFeature()->getCapture();
+    auto ftr = getFeature();
+    if (ftr != nullptr) return ftr->getCapture();
+    else return nullptr;
 }
 
 FrameBasePtr FactorBase::getFrame() const
 {
-    assert(getCapture() != nullptr && "calling getFrame before linking with a capture");
-    return getCapture()->getFrame();
+    auto cpt = getCapture();
+    if(cpt != nullptr) return cpt->getFrame();
+    else return nullptr;
 }
 
 SensorBasePtr FactorBase::getSensor() const
 {
-    assert(getCapture() != nullptr && "calling getSensor before linking with a capture");
-    return getCapture()->getSensor();
+    auto cpt = getCapture();
+    if(cpt != nullptr) return cpt->getSensor();
+    else return nullptr;
 }
 
 void FactorBase::setStatus(FactorStatus _status)