Description of problem:
When running `dnf system-upgrade download --releasever 30 --setopt='module_platform_id=platform:f30'`, I get a traceback after the transaction check.

Version-Release number of selected component (if applicable):
$ rpm -qa | grep dnf
python3-dnf-plugin-leaves-4.0.6-1.fc29.noarch
python3-dnf-plugins-extras-common-4.0.4-1.fc29.noarch
dnf-data-4.2.2-2.fc29.noarch
python3-dnf-4.2.2-2.fc29.noarch
python3-libdnf-0.28.1-1.fc29.x86_64
dnf-utils-4.0.6-1.fc29.noarch
dnf-plugins-core-4.0.6-1.fc29.noarch
dnf-automatic-4.2.2-2.fc29.noarch
python2-libdnf-0.28.1-1.fc29.x86_64
python3-dnf-plugin-system-upgrade-4.0.4-1.fc29.noarch
dnf-4.2.2-2.fc29.noarch
libdnf-0.28.1-1.fc29.x86_64
python2-dnf-4.2.2-2.fc29.noarch
python3-dnf-plugins-core-4.0.6-1.fc29.noarch

How reproducible:
always on the single machine I tested.

Steps to Reproduce:
1. `dnf system-upgrade download --releasever 30 --setopt='module_platform_id=platform:f30'`
2. have a look at the output

Actual results:
After transaction check and test, during "Transaktion wird ausgeführt" (German for "Running transaction", see dnf/base.py:881), I get this traceback:

  File "/usr/lib/python3.7/site-packages/dnf/yum/rpmtrans.py", line 260, in callback
    self._elemProgress(key, amount)
  File "/usr/lib/python3.7/site-packages/dnf/yum/rpmtrans.py", line 303, in _elemProgress
    transaction_list = self._extract_cbkey(key)
  File "/usr/lib/python3.7/site-packages/dnf/yum/rpmtrans.py", line 244, in _extract_cbkey
    raise RuntimeError("TransactionItem not found for key: %s" % cbkey)
RuntimeError: TransactionItem not found for key: rtkit

Expected results:
No traceback

Additional info:
I have no clue whether this actually is a real issue or just cosmetic stuff. dnf system-upgrade tells me later: "Download complete! Use 'dnf system-upgrade reboot' to start the upgrade." So it does not seem to know that it has thrown an error.