OhmSHRuleParser errors on incomplete Smalltalk method

[stlutz]

On a fresh subclass of OhmGrammarSmalltalkProxy, when writing the class-side serializedGrammar method, the error Error: ByteString called #basicNew: with invalid argument -1 is thrown once the text editor contains the following:

serializedGrammar
^ '

The error is caused because the following code always assumes the opening single quote to be closed in the end:

Ohm-S/packages/Ohm-Support.package/OhmSHRuleParser.class/instance/rangesIn.classOrMetaClass.workspace.environment.context..st

Line 14 in 98bf92f

grammarSource := aString copyFrom: offset + 1 to: (aString findLast: [:c | c = $']) - 1.