For Leveller 2.5 (and possibly the 2.6 alpha), there is a bug when
using plugin-based tools (e.g., custom brushes).
When multiple documents are open, Leveller will not correctly establish
the current heightfield object when the tool calls a function concerning
the document, such as retrieving its size. Instead, another open
document may be referenced. The plug-in tool will then confuse that
document with the actual current document. Brush tool plug-ins will not
be affected if they strictly use the LEV_TOOLBRUSHINFO structure.
Workaround: having only one document open at a time works, or, with the
target document active, choosing a different tool and then the desired
tool again may work.
There is currently no known workaround for a tool plug-in to correctly
obtain the current document's size.
My apologies for any inconvenience this error may cause, and I am
working to resolve it for 2.6.