copy() on Windows desktop fails

**Describe the bug**
Running command with `copy()` will unfocus the desktop before copying (sometimes moving focus to some window), throwing exception.

**Edit:** This bug is constant at times, but other times it simply doesn't happen, no matter how many times I follow the process. So you get streaks of fail and success, but I haven't pinned pinned down the cause.

I tried to isolate by cold booting and shutting down background apps. It still happened.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to desktop
2. Select some files
3. Run this:
```
[Command]
Name=Copy
Command="
copyq:
copy()
"
IsGlobalShortcut=true
Icon=\xf15b
GlobalShortcut=shift+f4
```
**Expected behavior**
Work like in Explorer or any window.

**Version, OS and Environment**
Discovered:
```
CopyQ Clipboard Manager v3.12.0-1-g9d084113
Qt: 5.13.2
Compiler: GCC
Arch: x86_64-little_endian-llp64
OS: Windows 10 (10.0)
```
Earliest known occurence:
```
CopyQ Clipboard Manager v3.11.0
Qt: 5.13.2
Compiler: GCC
Arch: i386-little_endian-ilp32
OS: Windows 10 (10.0)
```