Random null values in variables

[#1]

Dear Community,

I am relatively new QlikView user.

While developing my app I bumped into as believed unusual problem. I assigned functions into variable (in my case to get sub-string from selected field) and it worked perfect till yesterday... Randomly, the variable gets null value when different element of list box (on which functions are based) is selected. When I click clear button, the variable reset to correct value. Functions printed in text object always work fine. So I created clear all trigger on my list box when selected, but it didn't help. I will be very grateful for any idea which could solve the problem.

[#2]

There may be a space in the selected filed.

[#3]

Hi Anna,

Please could yo post the code?

[#4]

These are key part of my code:

text(num(year(datatmp))& '-' & num(Month(datatmp),'00')) as TextYM, this is my list box created in master calendar

vDateSelection =text(GetFieldSelections(TextYM))

vPreviousDate=FieldValue('TextYM', FieldIndex('TextYM','$(vDateSelection)')-1),
vPreviousMonth=right(FieldValue('TextYM', FieldIndex('TextYM','$(vDateSelection)')-1),2)

while function in vDateSelection always shows right values, vDateSelecion itself sometimes gets null, vPreviousDate and vPreviousMonth get null too.

[#5]

I owe an explanation to Qlik Community  After loading my app on server the problem, which I described simply disappeared. Therefore I assume it was general the problem with memory capacity; since I implemented vast number of variables it was hard to refresh values to all of them via desktop Qlikview.