ext6migrate: fix framework caching issue

ext6migrate: fix framework caching issue

in extjs 5/6 there is a caching issue, where they
save dom elements for reuse, but the garbage collector
can set them to null

when the framework now reuses the "cached" element it is null,
and any action on it produces an error, which breaks the site

for details see the forum link in the comment

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>