update get_continuous_events to only apply to systems that have them

Author: isaacsas

src/systems/callbacks.jl

@@ -1,7 +1,9 @@
 #################################### system operations #####################################
-get_continuous_events(sys::AbstractSystem) = SymbolicContinuousCallback[]
-get_continuous_events(sys::AbstractTimeDependentSystem) = getfield(sys, :continuous_events)
 has_continuous_events(sys::AbstractSystem) = isdefined(sys, :continuous_events)
+function get_continuous_events(sys::AbstractSystem)
+    has_continuous_events(sys) || return SymbolicContinuousCallback[]
+    getfield(sys, :discrete_events)
+end
 
 has_discrete_events(sys::AbstractSystem) = isdefined(sys, :discrete_events)
 function get_discrete_events(sys::AbstractSystem)