Electrum window dissapear completely on Linux while running in the background
Hi,
I am using Electrum 4.0.2 via AppImage on Linux, with Trezor Wallet with p2wpkh (SegWit native) addresses.

Today I noticed Electrum window dissapear completely from MATE's taskbar panel. However, Electrum processes were still running.
I killed processes and restarted Electrum again, also I enabled logging. Few hours later, Electrum dissapeared again - right after I clicked "View transaction on blockchain" on my History tab. Page successfully opened in my browser, but Electrum window is now completely gone.
Log is still being populated, as I have text editor notification that file has changed on disk, do you want to reload the file?
Processes running:

```
$ ps aux |grep electrum
pioruns    89321  2.2  1.1 5703448 184592 ?      Sl   15:23   4:52          |   \\_ /tmp/.mount_electrOI7vfa/usr/bin/python3.7 -s /tmp/.mount_electrOI7vfa/usr/bin/electrum
pioruns    89420  0.0  0.7 5060312 116288 ?      S    15:24   0:00          |       \\_ /tmp/.mount_electrOI7vfa/usr/bin/python3.7 -s /tmp/.mount_electrOI7vfa/usr/bin/electrum
pioruns    93960  0.0  0.7 5138016 126212 ?      S    18:55   0:00          |       \\_ /tmp/.mount_electrOI7vfa/usr/bin/python3.7 -s /tmp/.mount_electrOI7vfa/usr/bin/electrum
pioruns    94058  0.0  0.0   6080   892 pts/2    S+   18:56   0:00          |       \\_ grep --color=auto electrum
pioruns    89327  0.0  0.0  14216  2492 ?        Ssl  15:23   0:01 /home/pioruns/Electrum/electrum-4.0.2-x86_64.AppImage
```
Snipper from log - it begins with fiat query, I think that's when I clicked History tab:
```
20200717T175349.480598Z |     INFO | exchange_rate.Kraken | getting fx quotes for GBP
20200717T175349.743999Z |     INFO | exchange_rate.Kraken | received fx quotes
20200717T175349.745389Z |     INFO | gui.qt.history_list.HistoryModel | refreshing... reason: fx_quotes
20200717T175349.748349Z |    DEBUG | util.profiler | Abstract_Wallet.get_full_history 0.0027
20200717T175349.750533Z |    DEBUG | util.profiler | HistoryModel.refresh 0.0051
20200717T175349.750796Z |    DEBUG | util.profiler | AddressList.update 0.0000
20200717T175349.972019Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175350.889225Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175351.018612Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175355.156137Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175356.584476Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175358.344527Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175427.734298Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175434.175066Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175442.181584Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
20200717T175446.474981Z |     INFO | network | fee_histogram [[102, 116494], [86, 93555], [78, 142151], [76, 177146], [74, 96220], [71, 161182], [69, 182728], [68, 186781], [66, 247742], [65, 190117], [64, 350011], [63, 325184], [62, 296663], [59, 234014], [57, 625801], [56, 2585813], [55, 56941], [54, 62591], [53, 172244], [52, 68352], [51, 143497], [47, 679621], [41, 926271], [36, 863480], [30, 1045128], [22, 891991], [20, 1267917], [17, 1650435], [14, 1636925], [13, 1344823], [11, 2230623], [10, 1427310], [6, 1962210], [3, 2759281], [2, 2174864], [1, 15610316]]
20200717T175447.720091Z |     INFO | network | fee_estimates {25: 62037, 10: 62037, 5: 64748, 2: 68303}
```

But after that there is nothing, Electrum continues to run as normal, doing network fee estimates.
Opening AppImage does nothing at the moment, window will not appear. I have to kill all processes so I can start Electrum again.

System details from log:
```
20200717T142334.729262Z |     INFO | logging | Electrum version: 4.0.2 - https://electrum.org - https://github.com/spesmilo/electrum
20200717T142334.735790Z |     INFO | logging | Python version: 3.7.7 (default, Jun 28 2018, 19:01:02) 
[GCC 5.4.0 20160609]. On platform: Linux-5.6.0-0.bpo.2-amd64-x86_64-with-debian-10.4
```

I will be happy to provide further logs. Thanks
diff --git a/electrum/gui/qt/util.py b/electrum/gui/qt/util.py
index 4f1e8ec83..7e0f92885 100644
--- a/electrum/gui/qt/util.py
+++ b/electrum/gui/qt/util.py
@@ -1001,7 +1001,7 @@ def webopen(url: str):
         if os.fork() == 0:
             del os.environ['LD_LIBRARY_PATH']
             webbrowser.open(url)
-            sys.exit(0)
+            os._exit(0)
     else:
         webbrowser.open(url)
 
['electrum/gui/qt/util.py']