Do not crash when disabling problem reporter plugin

If we do no unregister ourselves, KTextEditor will try calling into the
destroyed notes provider.
1 job for !175 with work/unregisternotes in 60 minutes and 10 seconds
Status Job ID Name Coverage
failed #71393
linux kf5-qt5 qt5.12


Name Stage Failure
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
                                              14      10    71%   61,63-65
plugins/welcomepage/welcomepageview.cpp 26 25 96% 65
TOTAL 141713 64572 45%
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build: found 16332 matching files and directories

ERROR: Job failed: execution took longer than 1h0m0s seconds