Freemarker type mismatch on build queue

The symptom:

2009-04-22 12:56:29,747 ERROR [TP-Processor2] [runtime]
Expected hash. currentlyBuilding evaluated instead to freemarker.template.SimpleScalar on line 121, column 35 in fragments/showBuildQueue.ftl.
The problematic instruction:
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==> if !currentlyBuilding.executableElasticImages.isEmpty() [on line 121, column 29 in fragments/showBuildQueue.ftl]
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The cause:

On line 114 f the showBuildQueue.ftl file we set currentlyBuilding to ' ' if it isn't found, but forget on line 121 to check if it has any content before calling the methods on it.

As an aside, I am not sure if we really should be setting what is supposed to be another object (something that isn't a string) to an empty string rather than just leaving it null...