There is a problem with this "fcitx-mozc".

(P.S.)
I rewrote the procedure in an easy-to-understand manner.

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1. Start the text editor (pluma). This is an important point.
2. The keyboard icon (jp) is displayed to the right of the top bar.
3. Left click on the icon with the mouse.
4. Then a dialog screen will appear in which no characters are displayed.
5. After that, it will not accept keyboard and mouse input.
6. Because there is no way, I forcibly turned off the power.

Changing the input method to "ibus-mozc" will eliminate the problem.

(P.S.)

I reinstalled the system.
The same fault occurs fixedly.

I saw the log and there was a crash dump.
I think that you can send more information than the previous report.

I reported it by doing the following.
$ ubuntu-bug /var/crash/_usr_bin_fcitx-dbus-watcher.1000.crash

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/var/log/apport.log.1

ERROR: apport (pid 3904) Mon May 7 21:23:47 2018: called for pid 3826, signal 3, core limit 0, dump mode 1
ERROR: apport (pid 1638) Mon May 7 21:27:35 2018: called for pid 1255, signal 6, core limit 0, dump mode 1
ERROR: apport (pid 1638) Mon May 7 21:27:35 2018: executable: /usr/bin/fcitx-dbus-watcher (command line "/usr/bin/fcitx-dbus-watcher unix:abstract=/tmp/dbus-nz01H9srUR,guid=0190960852f560e8cdc8c6125af0460f 1251")
ERROR: apport (pid 1638) Mon May 7 21:27:38 2018: gdbus call error: Error connecting: Could not connect: Connection refused

ERROR: apport (pid 1638) Mon May 7 21:27:38 2018: debug: session gdbus call:
ERROR: apport (pid 1638) Mon May 7 21:27:38 2018: wrote report /var/crash/_usr_bin_fcitx-dbus-watcher.1000.crash
ERROR: apport (pid 3102) Mon May 7 22:17:56 2018: called for pid 2849, signal 3, core limit 0, dump mode 1

(P.S.)

[Additional notes]

The keyboard is connecting the Japanese keyboard.

The Japanese input method setting is as follows.
1. キーボード-日本語 (Keyboard-Japanese)
2. Mozc

So,
1. The top bar indicator is "keyboard icon +jp" → IME off
2. The top bar indicator is "あ" → IME on

Then,
A stall occurs when "IME off": "keyboard icon +jp".
It is normal when "IME on": "あ".

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By the way,
For the display when the text editor is not activated,
The top bar indicator is "keyboard icon".
If you left-click here, no fault will occur.

When the text editor is running and the Japanese input method is off,
The top bar indicator is "keyboard icon +jp".
If you left-click here, a stall will occur.

When the text editor is running and the Japanese input method is on,
The top bar indicator is "あ".
If you left-click here, no fault will occur.

I have a similar problem with pluma, and vim in a terminal, on my ubuntu mate 18.04, switching from my default language to mozc with Japanese.
Also, with AbiWrite I have the impression the behaviour is a bit different (but I've only tried once): I can switch languages but when I type the text appears on a tiny window outside the editor and there seems to be no way to insert it in the editor window.
I can try more tests, but given the necessary "hard reboot", I fear for the integrity of my system.
Perhaps switching languages in an "empty" (ie with no active text editor) workspace?
I'd really like to see this fixed.
ありがとうございます。