Measure: Fix crash when removing referenced element

Also improve error messages
Fixes #16183

src/Mod/Measure/App/MeasureLength.cpp

@@ -97,12 +97,6 @@ void MeasureLength::parseSelection(const App::MeasureSelection& selection)
 
 
 App::DocumentObjectExecReturn* MeasureLength::execute()
-{
-    recalculateLength();
-    return DocumentObject::StdReturn;
-}
-
-void MeasureLength::recalculateLength()
 {
     const std::vector<App::DocumentObject*>& objects = Elements.getValues();
     const std::vector<std::string>& subElements = Elements.getSubValues();
@@ -115,7 +109,7 @@ void MeasureLength::recalculateLength()
         App::SubObjectT subject {objects.at(i), subElements.at(i).c_str()};
         auto info = getMeasureInfo(subject);
         if (!info || !info->valid) {
-            continue;
+            return new App::DocumentObjectExecReturn("Cannot calculate length");
         }
 
         auto lengthInfo = std::dynamic_pointer_cast<Part::MeasureLengthInfo>(info);
@@ -123,8 +117,10 @@ void MeasureLength::recalculateLength()
     }
 
     Length.setValue(result);
+    return DocumentObject::StdReturn;
 }
 
+
 void MeasureLength::onChanged(const App::Property* prop)
 {
     if (isRestoring() || isRemoving()) {
@@ -132,7 +128,8 @@ void MeasureLength::onChanged(const App::Property* prop)
     }
 
     if (prop == &Elements) {
-        recalculateLength();
+        auto ret = recompute();
+        delete ret;
     }
 
     MeasureBase::onChanged(prop);