Fixes #4051- App crashes in explore

[ashishkumar468]

Description (required)
App crashes in explore

• Fixes App crashes while back press in explore #4051

What changes did you make and why?

• Handled possible NPE in ExploreListRootFragment

Tests performed (required)

Tested betaDebug API 27

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[codecov-io]

Codecov Report

Merging #4052 (464e3fc) into master (c93ab86) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #4052      +/-   ##
============================================
- Coverage     10.65%   10.61%   -0.05%     
  Complexity      476      476              
============================================
  Files           343      343              
  Lines         12821    12871      +50     
  Branches        995      997       +2     
============================================
  Hits           1366     1366              
- Misses        11387    11437      +50     
  Partials         68       68              

Impacted Files	Coverage Δ	Complexity Δ
...e/nrw/commons/explore/ExploreListRootFragment.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../main/java/fr/free/nrw/commons/utils/DialogUtil.kt	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...n/java/fr/free/nrw/commons/auth/LoginActivity.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...w/commons/contributions/ContributionsFragment.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...commons/nearby/fragments/NearbyParentFragment.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...upload/mediaDetails/UploadMediaDetailFragment.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)

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