FileHandlers: Fall back when _getSystemHandler() fails

Continue to Zotero.launchFile() as intended.

https://forums.zotero.org/discussion/113179/pdfs-will-not-open-in-system-reader-when-double-clicked

chrome/content/zotero/xpcom/fileHandlers.js

@@ -90,6 +90,7 @@ Zotero.FileHandlers = {
 		// If there are handlers for this platform and this reader type...
 		if (handlers) {
 			// First try to open with the custom handler
+			if (handler) {
 				try {
 					for (let [i, { name, open }] of handlers.entries()) {
 						if (name.test(handler)) {
@@ -102,6 +103,7 @@ Zotero.FileHandlers = {
 				catch (e) {
 					Zotero.logError(e);
 				}
+			}
 
 			// If we get here, we don't have special handling for the custom
 			// handler that the user has set. If we have a location, we really

test/tests/fileHandlersTest.js

@@ -109,5 +109,39 @@ describe("Zotero.FileHandlers", () => {
 			readerOpenSpy.restore();
 			getSystemHandlerStub.restore();
 		});
+
+		it("should fall back when handler is set to system and we can't retrieve the system handler", async function () {
+			let pdf = await importFileAttachment('wonderland_short.pdf');
+			let wasRun = false;
+			let readerOpenSpy = sinon.spy(Zotero.Reader, 'open');
+			let launchFileStub = sinon.stub(Zotero, 'launchFile');
+			Zotero.FileHandlers._mockHandlers = {
+				pdf: [
+					{
+						name: new RegExp(''),
+						async open() {
+							wasRun = true;
+						}
+					}
+				]
+			};
+
+			// Set our custom handler to something nonexistent,
+			// and stub the system handler to something nonexistent as well
+			Zotero.Prefs.set('fileHandler.pdf', 'system');
+			let getSystemHandlerStub = sinon.stub(Zotero.FileHandlers, '_getSystemHandler');
+			getSystemHandlerStub.returns(false);
+
+			await Zotero.FileHandlers.open(pdf, { location: {} });
+			assert.isFalse(wasRun);
+			assert.isFalse(readerOpenSpy.called);
+			assert.isTrue(launchFileStub.called);
+			assert.notOk(Zotero.Reader.getByTabID(win.Zotero_Tabs.selectedID));
+			assert.isEmpty(Zotero.Reader.getWindowStates());
+
+			readerOpenSpy.restore();
+			launchFileStub.restore();
+			getSystemHandlerStub.restore();
+		});
 	});
 });