Check if Mode is a str

If you initialize the gateway without parameters, mode is *not* a string
thus causing a "no such method" error when checking for the ":force"
option.

Author: elevans

src/imagej/__init__.py

@@ -1207,7 +1207,11 @@ def init(
 
         ij = imagej.init("sc.fiji:fiji", mode=imagej.Mode.GUI)
     """
-    force = mode.endswith(":force")
+    if isinstance(mode, str):
+        force = mode.endswith(":force")
+    else:
+        force = False
+
     if force:
         mode = mode[:-6]