fixed variable type change during text import bug #58298
Changing the variable type during the import dialog resulted
in a crash. The reason is that the import data is stored as
strings in a treeview and converted to union value depending
on the variable type. When the variable type is changed, there
is a discrepancy between then caseproto in the reader and the
variable. This results in an assertion.
As a fix I store the dictionary at the time of creation of the
casereader. When a variable is changed i reinitialize the
casereader.