Unable to Reproduce
I was using the Report Designer for a while know and never had any issues until now. Since 11.03.2019 Jaspersoft Studio throws a "java.lang.NullPointerException - An error has occurred. See error log for more details" -> Error Log: "java.lang.NullPointerException"
I've also researched in the web about this issue and i am not the only one with this kind of issue. Sadly there are no working bug fixes on the forums.
Now you might be asking: Did i change anything on the file, that i want to open? Yes i did but the Changes where minor (just making a textfield bigger). And also, a college is able to open the same file with the JasperReports Designer.
I've reinstalled Jaspersoft Studio multiple times now with no working result.
I hope I've given you enough Information to look into this issue.