view-switcher-title: Null-check view switcher in dispose()

If it's NULL, we can't get the stack anymore. This can happen if dispose()
is called multiple times.
11 jobs for wip/exalm/fixes2 in 19 minutes and 31 seconds (queued for 1 minute and 56 seconds)
latest
Status Job ID Name Coverage
  Build
passed #258109
librem5
api-visibility

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passed #258111
librem5
build-debian-clang

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passed #258110
librem5
build-debian-gcc

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passed #258112
librem5
build-fedora-gcc

00:04:01

 
  Test+Docs
passed #258113
librem5
build-gtkdoc

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passed #258114
librem5
unit-test

00:03:25

43.7%
 
  Package
passed #258115
librem5
package-debian-buster

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passed #258116
librem5:arm64
package-debian-buster:arm64

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passed #258117
flatpak
package-flatpak

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  Test Package
passed #258118
librem5
autopkgtest-debian-buster-package

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passed #258119
librem5
lintian-debian-buster-package

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