Distribution upgrade from 14.04 to 16.04 stuck installing pepperflashplugin-nonfree (amd64) - Ask Ubuntu

what should do? have "distribution upgrade" window open. has green check marks along "preparing upgrade", "setting new software channels" , "getting new packages." on "installing upgrades" step, , hanging on "preparing pepperflashplugin-nonfree (amd64)". there expandable/collapsible terminal @ bottom of "distribution upgrade" window, no text visible (it black), , when click on it flashes.

my screenshot application not open (perhaps because upgrade in progress?), can still open chrome. should do? should restart machine? in upgrade warned not turn off machine until upgrade complete. should hanging part way through?

edit: screenshot tool no longer opens, apparently gimp does. here screenshot:

when click on "terminal" black rectangle flashes (without changing size, i.e. not open up). if right click on "distribution upgrade" bar @ top of window, minimize, maximize , resize options greyed out.

edit: here output ps aux | grep flashplugin:

root     15540  0.0  4.4 287072 266080 pts/12  ss+  00:08   0:01 /usr/bin/dpkg --force-overwrite --status-fd 80 --unpack --auto-deconfigure /var/cache/apt/archives/pepperflashplugin-nonfree_1.8.2ubuntu1_amd64.deb /var/cache/apt/archives/python-pymysql_0.7.2-1_all.deb /var/cache/apt/archives/cabextract_1.6-1_amd64.deb /var/cache/apt/archives/ttf-mscorefonts-installer_3.4+nmu1ubuntu2_all.deb /var/cache/apt/archives/snap-confine_1.0.43-0ubuntu1~16.04.1_amd64.deb /var/cache/apt/archives/ubuntu-core-launcher_1.0.43-0ubuntu1~16.04.1_amd64.deb /var/cache/apt/archives/unity-gtk-module-common_0.0.0+15.04.20150118-0ubuntu2_all.deb /var/cache/apt/archives/unity-gtk2-module_0.0.0+15.04.20150118-0ubuntu2_amd64.deb /var/cache/apt/archives/unity-gtk3-module_0.0.0+15.04.20150118-0ubuntu2_amd64.deb /var/cache/apt/archives/vlc-plugin-samba_2.2.2-5_amd64.deb /var/cache/apt/archives/libcurl3_7.47.0-1ubuntu2.1_amd64.deb /var/cache/apt/archives/whoopsie_0.2.52.1_amd64.deb /var/cache/apt/archives/libwhoopsie0_0.2.52.1_amd64.deb /var/cache/apt/archives/libss2_1.42.13-1ubuntu1_amd64.deb adrian   17739  4.7  4.3 3027800 261616 ?      sll  00:29   0:52 /usr/lib/chromium-browser/chromium-browser --ppapi-flash-path=/usr/lib/adobe-flashplugin/libpepflashplayer.so --ppapi-flash-version --enable-pinch adrian   17757  0.0  0.9 441452 57596 ?        s    00:29   0:00 /usr/lib/chromium-browser/chromium-browser --type=zygote --ppapi-flash-path=/usr/lib/adobe-flashplugin/libpepflashplayer.so --ppapi-flash-version adrian   17761  0.0  0.1 441452 11176 ?        s    00:29   0:00 /usr/lib/chromium-browser/chromium-browser --type=zygote --ppapi-flash-path=/usr/lib/adobe-flashplugin/libpepflashplayer.so --ppapi-flash-version adrian   19670  0.0  0.0  18372   988 tty1     s+   00:47   0:00 grep --color=auto flashplugin 

got warning -- don't know whether related earlier problems:

this related bug #1632870.

i'd suggest enable canonical partner , run:

sudo apt purge pepperflashplugin-nonfree sudo apt update sudo apt install adobe-flashplugin 

the adobe-flashplugin package install updated version of libpepflashplayer.so. install libflashplayer.so e.g. firefox.