qt: fix qt-error-dialogs inherit type

Running the (3.0) code with --no-qt-error-dialogs asserts.
20 jobs for master in 13 minutes and 10 seconds (queued for 11 seconds)
Status Name Job ID Coverage
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passed android-arm #800172
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passed android-arm64 #800173
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passed android-x86 #800174
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passed android-x86_64 #800175
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passed debian #800163
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passed ios-arm64 #800168
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passed ios-armv7 #800169
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passed ios-simulator-x86_64 #800170
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passed macos-arm64 #800167
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passed macos-x86_64 #800166
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passed raspbian-arm #800165
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passed snap #800164
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passed tvos-arm64 #800171
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passed uwp-arm64-libvlc-llvm #800162
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passed uwp64-libvlc-llvm #800161
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passed wasm32-emscripten-build #800176
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passed win32 #800157
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passed win64 #800158
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passed win64-llvm #800159
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passed win64-ucrt-llvm #800160
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