Fix Explorer with Layout Builder enabled

[MurzNN]

Rerolled patch from #733 (comment)
Closes #1088

[codecov]

Codecov Report

Merging #1144 (9885d96) into 8.x-3.x (8418bb7) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             8.x-3.x    #1144      +/-   ##
=============================================
+ Coverage      78.72%   78.75%   +0.02%     
- Complexity      1139     1142       +3     
=============================================
  Files            192      192              
  Lines           3027     3031       +4     
=============================================
+ Hits            2383     2387       +4     
  Misses           644      644              

Impacted Files	Coverage Δ	Complexity Δ
src/Plugin/GraphQL/Schemas/SchemaPluginBase.php	79.75% <100.00%> (+0.50%)	53.00 <0.00> (+3.00)

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[bronius]

Hi - Thanks for the patch. I don't know enough about this and my current configuration to speak too informedly here just yet, but at the moment, I confirm the patch applies but does not resolve the original issue in #733 (Console reads: Uncaught TypeError: Cannot read property '__schema' of undefined, and Explore screen is blank) in at least my use case. In my use case, when I click into /admin/config/graphql/schemas I see only "Default : GraphQL Core" listed.

[Luigisa]

This request has been included in the new versions of the 3.x module.
Thanks @MurzNN and @bronius