concatenate with a varying range of cells
 

I have check boxes in a worksheet, if the box is checked a true value is
assigned. If true, it takes the information from the column next to it and
puts it into another column descending in order. I would like to take all of
the "true" values and put them into one cell. My problem is the rows of my
data changes. When I identify the cell range, it brings back an error if
there is nothing in the cell or the #NUM!. Is there a formula to ignore if
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