src/goVulncheck.ts: only run vulncheck for active editors of go files

Fixes golang/vscode-go#2223.

Change-Id: I8812b85d564e7f58880bcd94094bb4114cd36f3d
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/403774
TryBot-Result: kokoro <noreply+kokoro@google.com>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/src/goVulncheck.ts b/src/goVulncheck.ts
index 756e1af..fe173b4 100644
--- a/src/goVulncheck.ts
+++ b/src/goVulncheck.ts
@@ -43,14 +43,20 @@
 	private async runInternal() {
 		const pick = await vscode.window.showQuickPick(['Current Package', 'Workspace']);
 		let dir, pattern: string;
-		const filename = vscode.window.activeTextEditor?.document?.fileName;
+		const document = vscode.window.activeTextEditor?.document;
 		switch (pick) {
 			case 'Current Package':
-				if (!filename) {
+				if (!document) {
 					vscode.window.showErrorMessage('vulncheck error: no current package');
 					return;
 				}
-				dir = path.dirname(filename);
+				if (document.languageId !== 'go') {
+					vscode.window.showErrorMessage(
+						'File in the active editor is not a Go file, cannot find current package to check.'
+					);
+					return;
+				}
+				dir = path.dirname(document.fileName);
 				pattern = '.';
 				break;
 			case 'Workspace':