Your problem is unrelated to the web.allowForking setting.
You need to enable repo caching.

git.cacheRepositoryList = true

To be fair, it is related to but not caused by web.allowForking.

There appear to be a handful of bugs related to tracking fork state and not caching
the repository list.  The reason to cache the repo list is to avoid abusing the disk
to determine information which is static.  The argument against caching is creating/removing
repos from outside Gitblit.  If you plan to use Gitblit as a server then I recommend
using it's caching mechanism.  If you plan to use Gitblit as a viewer then it may make
sense to disable caching but you should also consider disabling it's server features
like forking.

I pushed a change for your NPE and also pushed another change for a related NPE.  Another
drawback of not using the repository cache is not having the fork network feature.
 Fork network (tree/graph) is demonstrated here:

https://demo-gitblit.rhcloud.com/forks/gitblit.git

While you could generate the fork network per-request without the cache it would be
expensive/abusive for the disk to cycle through all your repos to determine the graph.
 Now multiply that by my (known) worst-case user with ~7k repos.  :(

Thanks for the prompt response and the advice!

Fixes deployed in 1.2.1

Hi 

This still appears to be an issue in Gitblit Go 1.2.1 (it can be worked around by setting
git.cacheRepositoryList = true).

We get an error page in the GUI when clicking on a repository with the following options
set:
web.allowForking = true
git.cacheRepositoryList = false

Stack trace:
java.lang.NullPointerException
        at com.gitblit.GitBlit.getFork(GitBlit.java:1830)
        at com.gitblit.wicket.pages.RepositoryPage.setupPage(RepositoryPage.java:274)
        at com.gitblit.wicket.pages.RepositoryPage.onBeforeRender(RepositoryPage.java:526)
        at org.apache.wicket.Component.internalBeforeRender(Component.java:1069)
        at org.apache.wicket.Component.beforeRender(Component.java:1103)
        at org.apache.wicket.Component.prepareForRender(Component.java:2297)
        at org.apache.wicket.Page.prepareForRender(Page.java:1545)
        at org.apache.wicket.Component.prepareForRender(Component.java:2329)
        at org.apache.wicket.Page.renderPage(Page.java:912)
        at org.apache.wicket.request.target.component.BookmarkablePageRequestTarget.respond(BookmarkablePageRequestTarget.java:261)
        at org.apache.wicket.request.AbstractRequestCycleProcessor.respond(AbstractRequestCycleProcessor.java:105)
        at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1287)
        at org.apache.wicket.RequestCycle.step(RequestCycle.java:1358)
        at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1465)
        at org.apache.wicket.RequestCycle.request(RequestCycle.java:545)
        at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:486)
        at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:319)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1300)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:445)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
        at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:556)
        at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:227)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1038)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:374)
        at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:189)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:972)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
        at org.eclipse.jetty.server.Server.handle(Server.java:363)
        at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:483)
        at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:920)
        at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:982)
        at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:635)
        at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
        at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
        at org.eclipse.jetty.io.nio.SslConnection.handle(SslConnection.java:196)
        at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:628)
        at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
        at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
        at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
        at java.lang.Thread.run(Thread.java:662)

Fixed on master.
Thanks for the report!

Fix or enhancement released in v1.3.0