check if textbox exists vba (using name)

I am using Ms-Access and I created a userform which has a number of Textboxes on it. The boxes are named: Box1, Box2, Box3 ...

I need to loop through all boxes, but I don't know which is the last one. To avoid looping through all userform controls I thought of trying the following:

For i =1 To 20

If Me.Controls("Box" & i).value = MyCondition Then
    'do stuff
End If

Next i

This errors at Box6, which is the first box not found. Is there a way to capture this error and exit the loop when it happens.

I know I could use On Error but I 'd rather capture this specific instance with code instead.

Thanks, George