FileTreeContextMenu_Opening: Handle null revision

[mstv]

Fixes #10666

Proposed changes

• FileTreeContextMenu_Opening: Replace Validates.NotNull(_revision) with null-aware code
• ShouldEnableFirstItemDiff handle null
• Disable filterFileInGridToolStripMenuItem, too, if there is no selection
• Replace var by explicit types

Screenshots

Before

NRE

After

Test methodology

• manual

Test environment(s)

• Git Extensions 33.33.33
• Build 450e4c0
• Git 2.39.0.windows.1
• Microsoft Windows NT 10.0.19045.0
• .NET 6.0.12
• DPI 96dpi (no scaling)
• Microsoft.WindowsDesktop.App Versions

    Microsoft.WindowsDesktop.App 6.0.12 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Merge strategy

I agree that the maintainer squash merge this PR (if the commit message is clear).

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