SceneBuilder closes unexpectedly when an error occurs during FXML file loading

[Oliver-Loeffler]

SceneBuilder by default starts into the welcome page.
If one attempts to load either an existing project or a recent project, SceneBuilder will close in case of an error during file loading.
This is the case by:

• no longer existing recent projects
• attempting to load any unsupported files
• attempt to load a corrupted FXML file
• attempt to load a incompatible FXML file

Expected Behavior

SceneBuilder should keep the welcome page open and display the error (if it is one file) to the user.
The user should be able to obtain a stack trace for further analysis.
SceneBuilder shall not close after confirming the error, so that the user has the chance to load another file.

Current Behavior

SceneBuilder displays the error to the user.
SceneBuilder closes after the user confirms the file loading error.

Steps to Reproduce

1. start SceneBuilder
   1. attempt to open a no longer existing project from welcome page
   2. attempt to load a broken/unsupported FXML file via open project button

Your Environment

• This issue occurs on Windows, Linux and Mac.

Screenshots

N/A

[Oliver-Loeffler]

Closed with #585.