I just tried to open a Testplan in 2.10 that worked fine in JMeter 2.9.
The loading is interrupted by an NPE because 
org.apache.testbeans.gui.JSyntaxTextArea#setLanguage is called with null 
value.

I figured out the the custom plugin (based on TestBean) was missing to call

p.setValue(TEXT_LANGUAGE, "xml")

after creating a property with TypeEditor.TextAreaEditor.

I'd suppose to add a null check in the mentioned method and throw an 
IllegalArgumentException with a detailed error message, stating the the 
developer of the plugin needs to add a line like the above to it's 
BeanInfo class.

I would have done it already but wanted to ask if throwing an Exception 
in that case is wanted if it's better to log a warning and take the 
default (like in the case that the key isn't found in the 
textarea.properties)

Regards,
Michael