Ensure that the branch Tip is not null and throw a more intuitive exception than NulReferenceException if it is.

TODO: Need to document how to fix this and add the URI for it to the exception.

Author: asbjornu

src/GitVersionCore/LibGitExtensions.cs

@@ -50,16 +50,28 @@ public static SemanticVersion LastVersionTagOnBranch(this Branch branch, IReposi
 
         public static Commit FindCommitBranchWasBranchedFrom([NotNull] this Branch branch, IRepository repository, params Branch[] excludedBranches)
         {
+            const string missingTipFormat = "{0} has no tip. Please see http://example.com/docs for information on how to fix this.";
+
             if (branch == null)
             {
                 throw new ArgumentNullException("branch");
             }
 
+            if (branch.Tip == null)
+            {
+                throw new ArgumentException(String.Format(missingTipFormat, branch.Name));
+            }
+
             using (Logger.IndentLog("Finding branch source"))
             {
                 var otherBranches = repository.Branches.Except(excludedBranches).Where(b => IsSameBranch(branch, b)).ToList();
                 var mergeBases = otherBranches.Select(b =>
                 {
+                    if (b.Tip == null)
+                    {
+                        throw new InvalidOperationException(String.Format(missingTipFormat, b.Name));
+                    }
+
                     var otherCommit = b.Tip;
                     if (b.Tip.Parents.Contains(branch.Tip))
                     {
@@ -72,6 +84,7 @@ public static Commit FindCommitBranchWasBranchedFrom([NotNull] this Branch branc
             }
         }
 
+
         static bool IsSameBranch(Branch branch, Branch b)
         {
             return (b.IsRemote ? b.Name.Replace(b.Remote.Name + "/", string.Empty) : b.Name) != branch.Name;