[abenedik]

I have tried to reproduce the problem and was not able to do that.

I used the following xaml (as simple as possible):

Code:

<Grid Name="RootGrid">
        <Viewport3D Name="MainViewport" />
        <cameras:TargetPositionCamera Name="Camera1"
                                      TargetPosition="0 0 0"
                                      Heading="30" Attitude="-20" Bank="0" />
    </Grid>

and the following code behind:

Code:

this.Loaded += delegate (object sender, RoutedEventArgs args)
            {
                var boxVisual3D = new Ab3d.Visuals.BoxVisual3D()
                {
                    CenterPosition = new Point3D(0, 0, 0),
                    Size = new Size3D(100, 40, 80),
                    Material = new DiffuseMaterial(Brushes.Green)
                };

                MainViewport.Children.Add(boxVisual3D);

                Camera1.FitIntoView();
            };

This did not throw any exception.
I have also tried to replace the BoxVisual3D with wireframe object and still it worked.

I have also checked the code for FitIntoView and did not find a possible cause of NullReferenceException there.

Could you please post the full stack trace for the exception (you can also send it by email).

Note:
The simple code above does not work well - because the TargetViewport3D property is not set in the TargetPositionCamera, the Camera is not correctly initialized at the time of the FitIntoView call - therefore the camera is not adjusted correctly. To fix that I advice to set the TargetViewport3D:

Code:

<cameras:TargetPositionCamera Name="Camera1"
                                      TargetPosition="0 0 0"
                                      Heading="30" Attitude="-20" Bank="0"
                                      TargetViewport3D="{Binding ElementName=MainViewport}"/>

It is also possible to call Camera1.Refresh() method before FitIntoView - this will search the visual tree and automatically find the Viewport3D (without setting TargetViewport3D). Because this will be done before the FitIntoView call, the FitIntoView will correctly adjust the camera. I will improve that in the next version.