Can I file current tax year return even though 1040x of previous year was not completed yet?
Yes, you can complete and file the 2019 tax return before the 2018 tax return is amended.
Where asked for the 2018 AGI use the AGI from the original 2018 tax return. If that AGI is rejected for e-filing then enter a 0 (zero) for the 2018 AGI.